Experience with gel-filled junction boxes
Well, there is an entire industry making what you want, so you would think it not too hard to find some solutions. But alas, it is quite difficult. The industry is geared toward large telecom companies with direct advertising to local telephone companies. I worked as a fiber optic tech for four years and have used quite a few different cases like what you're looking for, I'm assuming you're going for the smaller ones. For the most part, as I said, the cases don't get filled with a fluid gel. Most of them just seal incredibly well. The main players (as of six years ago, before I went back to school) were:

Coyote Splice Enclosures, from them I'd recommend the runt, it's quick to assemble and I've seen them sitting in water without taking any. The hardest thing with these is the cable wrapping takes practice to get right.

Tyco Splice Enclosures, they now go by TE, the "A" enclosure from their 450 series is a pretty awesome little dome. Fewer bolts and the entire cable entry gets a big sticky gel ring that seals it up tight. The cables just snap into the end plate and you screw two discs together to make the gel ring squish out around the cables and the dome, sealing them at the same time. They also make a nice line of copper enclosures (some that can be direct buried), which I have much less experience with.

The cases you've linked to seem quite nice too. They do look quite a bit less expensive, but messier. I can't comment on their effectiveness, but I do know a properly built fiber enclosure won't leak. The down side is the cost and the fact that you're not doing fiber in them, so the trays and other accessories likely won't be useful. Keep in mind many of these cases were designed for telecom manholes which, at least whenever I needed to get into them, were completely full of water.

As for acquiring these cases, try asking your local telephone company or look up some local fiber splicers, they might be willing to sell you some at their bulk rate. 

I hope this helps. I'd be interested to see what you come up with.

-Samuel