i guess the first question would be: have you ever worked with fiberglass
doing a false floor with subs in it isnt that hard, but can be tedious to explain. i would suggest going to sites like www.sounddomain.com or www.termpro.com and clicking on thier forum sections, then searching for somthing like "fiberglass spare tire" or somthing. do a lot of reading and youll learn a lot.
basically the steps are this:
1) remove the spare tire and mounting brackets for the jack and tire (with a neumatic grinder of some sort).
2) lay down either (a) mold release wax (b) duct or masking tape (c) aluminum foil (d) plastic tarp or (e) any combination of the above in the spare tire area and along the floor. make sure that it is steady and contures closly to the curves of the spare tire well.
3) create a square (doesnt have to be square, but i'll simplify it;)) frame out of 2x4s or similar to fit around the outside of the tire well. basically, this is the structure that the fiberglass will adhear to around the outside of the well. if you can imagine looking down from a birds eye view on the spare tire well, it would look like a circle. you want to construct a square that would fit around the outsides of the well. if you are using more than one sub, increase the width of the frame to compensate for the extra speakers diameters. if you are using for example 2 12"s, you might make the frame 30" wide and 15" high, so that the 12"s will fit without a problem. to increase airspace for the speakers, make the frame taller vertically. drill a small hole in one side of the frame to run speaker wire through later.
4) lay fiberglass down in the wheel well and smooth it up to the sides of the frame, so that it all becomes one piece. after 2-3 layers of this, you should be able to remove it all from the car. lay down another 3-4 layers untill the fiberglass is about 1/3"-1/2" thick. i could write pages on how to lay fiberglass, but youre better off researching it yoursel
5) create a false floor panel that will fit over the top of the fiberglass/frame piece. cut holes in it to mount the subs in.
6) with the tire well section in the car, lay the false floor on top and drill holes to mate the two pieces. lay down a bead of wood glue and screw the pieces together. carpet/vinyl/paint and screw the subs into place.
