We've done this many time in other vehicles, (speaker boxes, gun safes, fuel tanks, etc...), and it works out great. One thing to keep in mind though, you shouldn't mount it to the frame and the body, because the body is not solid mounted and so it will flex different from the frame (this is bad, lol). We build a frame similar to what was posted, (out of 1.5" angle), but with wider wings at the top to span out the weight better, then we cut the hole and drop it in from the top. We sheet the whole thing from inside, weld and seal it up, and it becomes part of the car. If it were me, I'd nab the plastic floor part (with the lid piece) from a 2ng gen, from the wrecking yard, and start from there.