[tubbedII]

A false floor is simply a box that takes up the entire floor board and is built as shallow as possible to retain as much cargo space possible. You'd make a top plate that covers the entire box that's about 2" or so above the speaker to allow for speaker excursion, but the top plate would be the exact shape of the side panels to look like it's the floor...kinda like raising the bed floor when laying frame but doing it for a speaker box. You wouldn't need any special tools other than the typical box building tools.

In your situation with those speakers, your floor would be "raised" about a foot (7.5" speaker + 1" air gap between speak and bottom of box + 2" excursion + 1.5" of mdf). That's a lot of cubic space, but with that you could also mount your amps on the side of the box under the top plate so it's all hidden.

This probably still isn't the most ideal situation based on what your going for, but your speakers are a little limiting being so big.