What do I need for a portable 12V deep cycle battery?
900wH!  Very nice.  Should be workable.

You need to decide on material for the case, 

Plastic - Can be nonconductive, strong, light, your choice of material properties, tools can be expensive.  Material tends to be cheap.  Easy to work with.  Can be flammable.

Plastic definitely has its merits.  Careful which ones you pick but you can probably make an excellent, robust, middleweight and nonconductive case.  You're probably working with sheet plastic in the DIY pricerange but you can produce simple curves in most materials, which can make waterproofing easier.

Metal - Conductive, strong, light, Your choice of properties for toughness, hardness, etc, corrosion can be an issue, tools can be expensive for fast work.  Can be expensive.  Easy to work with.

If you go with metal, you've got a wide variety of sheet and barstock available, as well as a robust array of hardware and fittings.  If you're willing to put in the time, you can go a long way with just a drill, hacksaw, tinsnips, pliers and a file.  If weight is an issue, you can probably build a lighter metal case than plastic, if mechanical properties are equal.

Wood - Not particularly conductive on its own.  Flammable absorbent, heavy, tough.  Easy to work with.  Cheap.

If you go with wood you'll probably have to put some effort into figuring out how to make it light and adequately water resistant and still strong.  For electrical related purposes, wood isn't common except for temporary stuff or finish.  That said it wouldn't be the end of the world and you could be the guy with the burled mahogany battery bank.  Probably best to have an inner casing of metal.

I like lightweight myself, so I'd go with an aluminum or steel frame/roll cage.  I don't have resources for aluminum welding, so if it was aluminum I'd build a heavy duty frame from angle and box aluminum.  I'd figure out how strong it needed to be and double or triple that and then go selecting materials.  Once the frame is designed, I'd figure out how best to build the casing.  If you really want to go light there are fibreglass and even carbon fibre reinforced plastics.  If you need to shed heat, aluminum is great.

2 cigarette sockets will not be a problem.  You can either scavenge them from something (consider upgrading the wire while you're at it) or buy ones meant to mount to a case.  Have an inline fuse and/or DC circuit breaker on each socket.

Including a solar panel would not be a problem due to the ease of charging lead acid batteries.

2 USB outputs is not a problem either.  You'll have to make a faceplate, but pretty much any automotive USB adapter will do if you want to scavenge, or boards are readily available online, so I'd recommend finding two 2.1A Iphone enabled chargers rather than 1 with 2 ports, probably 2 ports each, and consider throwing an inductive charger in if you want it.  

Automotive voltage, power and current meters are readily available.  I would try to find a single device rather than multiple and look for low power consumption, which isn't particularly clear on many products.

A master disconnect (on/off switch for the whole thing) is definitely a good idea.

As far as a charger port, you could have a smaller door on the side or some such that opens to reveal external battery posts, and disconnect the posts when the door is closed, or you could build in some sort of plug, but then you'll also have to build a plug onto your charger.  You could also build a charger into your box, so you could plug it into the wall and let it charge itself.

If you want, it would also be easy enough to include an inverter so you could pull an amount of 120V power off of your device when you wanted to.

As far as what you might want to add other than that, definitely read up on lead-acid batteries as @Nick Alexeev suggests.  They contain acid, emit gases, and have other concerns, and you'll have to learn enough about them to build your case accordingly. You need to protect the battery physically from the outside world, safely discharge gases if necessary, protect the battery from the case, from short circuit, and of course protect the outside world from the battery, so think about how you're positioning things, and mount them securely.  Some of your components will need to shed heat, so you'll need to make sure they're adequately cooled.  

Cooling - You may need a fan if you include an inverter or a solar panel, or even if you just expect the device to get very hot in direct sunlight.  Nothing else discussed here is likely to produce a large amount of heat.  How large of a fan you might need depends on how much heat you're producing.  a 300W inverter that's 90% efficient will produce about 30W of heat at full load, so in that case, a small computer-type fan is likely just fine, and might be part of the inverter anyway.  Because the battery type you choose might vent dangerous gases, and positive pressure can be an advantage to prevent water intrusion, you may want a fan.  If you're not producing that much heat, and you're not using it for something else, it's just wasted power.    

Batteries - It is important that you read up on the exact battery and chemistry that you intend to use when it comes to large capacity potentially dangerous batteries, and for that reason I won't elaborate too much, other than to say, the battery you are planning to use can vent explosive gases, so you should figure out at the least what conditions cause venting, where they are vented from, and any datasheets that include engineering advice (like how much of an air channel is required when the battery is built into a device.).  

Also circuit protection.  Some of these devices if you're salvaging parts or buying boards online will include circuit protection, but I'd probably throw in fuses for each individual device at the very least as well as a main fuse.  If they were priced well enough I'd use DC breakers for the main shutoff and cigarette socket switches.  Fuses and circuit breakers go in series in a circuit and break the circuit when their trip conditions are met, so for small fuses, automotive inline fuse holders are probably a good option for you or maybe a case-mount fuse block if you want external access.  I would personally tend to put protection on the positive side of the circuit, although I'm not sure there is a worldwide convention for this.  

Be aware that busbars and terminal strips exist

If you're going to be plugging it into the wall, all nonconducting metal parts exposed to the user and all nonconducting metal parts of substantial size should be grounded.  If the cord is integral provide strain relief.

If you want to be able to boost your car in emergencies ensure it is adequately easy to access the battery terminals.

Most of your work will be building the box itself and putting holes in it and mounting to it, so some suggestions for the box:

Think about how to make it light and strong with every decision.  Think about how it will be carried, how dirty it will get, the environments it will be in (dust, flammable dust, no-airflow, flammable gas, corrosive gas, water(steam, water vapor, submersion, falling water, pressurized water), chemicals all have their own concerns).  At the very least you won't have too much trouble making one that is resistant to falling water and can protect devices plugged in if you want.  Just don't forget that water splashes upwards from the ground too if you go that way.

Do not work with line voltage (for ex. adding a built-in charger) unless you are certain you understand what you are doing.  Even 120v can stop your heart.  An unsafe design could stop someone else's.  Learn first, then build.  Low voltages and currents are safer to work with.  

Also of note is that it is important that when you select your parts, even something as simple as a switch or connector, you check to make sure they are rated for the voltage, the current, the temperature (for hot or cold environments), efficiency and mounting requirements at the least.