start off with the floor. look under the car and see where you can make mounting points. go get some 2x4s, 1/2" or 3/4" lag bolts, washers, and nuts (however long of bolts you need) and use the 2x4s, and bolt them to the floor. use 2x4s to get a level surface, and then put a piece of 3/4" mdf and glue and screw it to the bolted down 2x4s. you now have a flat surface to work with, and start building the box and slide it in, or however you like.
i put some liquid nails under the 2x4s, so the 2x4s arent directly touching the metal body, might help with small annoying rattles/squeaks that might occur after a while. just an idea.
heres some pics of the wall we built in my 97 explorer (not in anymore)
see the bolts?
leveling up the floor