Want to build a trap door in the guest bedroom for easier furnace access
I agree with @ecnerwal about planning for the future. 

If that's too big a plan, and as an alternative to cutting a whole new access hatch, you might just do some light excavating between your access and the filter to turn 30 minutes of misery into 10 minutes of misery. If your dirt is horrible clay, rent a light jackhammer with a shovel attachment.

Having said that, if you still want a hole in your guest bedroom floor, the closet is an ideal place for it, but of course your floor joists might not cooperate. 

Wherever you end up putting the hatch, the basic plan would look like this to me: 


figure out if you have enough space between the joists to make a reasonable hatch. 24" would be the minimum I'd consider. If your floor joists are on 16" centers, you'll have to put headers (doubles of whatever the floor joists are) at each end of the hole and cut out the middle joist. (If you google, "headers for skylight", you'll get good images of what I'm on about.) 
drill holes from below in opposite corners, close to the inside of the joists as you can, to mark 2 of your corners. Then, from above, drill the other 2 corners of the rectangle perpendicular/parallel to the flooring. Hopefully, these holes make a rectangle that's also approximately perpendicular/parallel to your joists. The underlying idea is that you really want your hole to be as close to the joists as possible without going over the joists themselves. If you need to cheat, make the hole smaller and still cut parallel to the finish floor. You'll need to add extra support later, but that's not a big deal. 
make a note of the thickness of your finish floor and subfloor combined.
mark the rectangle 1/2" smaller on each side from the drill holes. (Masking tape and a marker might make the line easier to see.)
assuming you're competent plunge cutting with a circular saw, set the depth of cut to just a hair deeper than your combined floor thickness and go ahead and cut. Don't let the saw kerf go past the confines of the rectangle. (Don't use your favorite circular saw blade -- you'll probably hit nails. Which reminds me to mention eye/ear protection.)
finish the corners with a jigsaw, reciprocating saw, or oscillating tool. 
if the subfloor of the bit you've cut out is in good condition, it can stay. Otherwise, get some 3/4" plywood and cut it a bit smaller than the hole. Either way, attach the hardwood by screwing through from the underside.
if you have joists on 16" centers, now is the time to cut the middle joist and install headers. Cut the middle joist 1-1/2" back from the edge of the hole so that the headers will be half in and half out with respect to the sides of the hole. (Reciprocating saw would be the weapon of choice for this, as you'll be able to cut the joist almost all the way without too much damage to the subfloor. Jigsaw would be next best, but that would involve some chisel work.) Hopefully the existing floor joists are between flush and 3/4" of the sides of the hole. 
if needed, headers should be double thickness of whatever the existing floor joist is. If your joists are actual 8", then doubled 2x8s are close enough. Don't make the headers much smaller, though. Use double joist hangers (4 in total) to connect the headers to the existing joists. Use a single joist hanger (2 in total) to connect the old joist to the back of the headers.
now you need to add material for the hatch cover to sit on. 2x4s can run down the length of the old joists. Make them flush with the top of your joist. 4 or 5 3" screws per side seems about right. If you're re-using your subfloor, the height will be perfect. If you substituted some ply, then you'll have to shim on top to get the floor level right. If there's not at least 3/4" of the hatch cover bearing on this support, add another layer.
for the hatch cover support that is perpendicular to the joists, either your new header will do or you'll run a 2x4 on the flat, half in and half out of the hole. Attach it well to the joists at both ends and screw up into the subfloor. (But don't expect the subfloor screws to be very good.) This might need shimming like the others.
add a handle of some description to your hatch. Commercial ones exist, though a very basic one is to drop a long (6-8") timber bolt (distinct from a carriage bolt in that there's no square section under the head) through a slightly oversized hole. Put a nut right at the end and you'll have a bolt that usually falls down into the hole and can be picked up when needed.
add trim around the hatch cover (like a thin door casing), throw it on the hole and you're done.


Whew! After all that, I'm guessing that a bit of digging sounds pretty good. Don't hesitate to comment if anything needs clarification.