MDF and fiberglass. Built a 3" mdf frame (make it as high as you need for clearence and proper air space) which sits on the floor and supports the box.

Fiberglass the holes from inside the frame where the factory styrofoam inserts were located so I can get the shape and utilize as much space as possiable.

Top is MDF, it is flat and can support as much weight as you can fit in the trunk, just screws down to the frame. each seperate box weighs less than the sub that I put in it.
Fasten the 2 seperate boxes together with whatever you want. I used thin flat steel bracket in 2 locations so there is no movement.

Now depending on how you do the wiring all depends on how easy the removal will be. If you have the amp located where I do you should not have to take it out. For removing the boxes it is just a matter of unscrewing those 2 little brackets, undo the speaker wire to each and pull them out.

The part of the floor that is visiable is 1 thin peice of panneling wrapped with whatever you want (carpet, vinyl, anything). very light and easly removable. Just sits flat ontop of the boxes and around the subs. The speaker covers I made fasten to this part and not to the individual boxes. They are spaced high enough so that the sub will not hit it and allow for air movement. The design on mine are cut out allowing for more air movent also.

You can make this system as easy or as hard as you want to be removed. Mine can be removed in under 2 min if the hatch is already open and I have a screwdriver in hand. comes out in 3 peices.