ok well here is the deal. drop the huge box on the ground. cut the port that you made off it, throw down 10 layers of fiberglass where the back seats go, install the port somewhere convienent, put the back seats in your garage, cut the solo's square baffle (cut them out of your existing box dummie!) fleece over the baffle\port and resin\carpet that. you will get the same cubic feet (if its too large, then fill it with some wood until you get your desired size.
always build fiberglass enclosures too big. you can always make it smaller, but its not so eas to make it larger.
this way you can bolt it from the inside, you can use some security screws for the subs, and you can make a fiberglass cover for your amp so people cant steal that too... people WILL take hammers\crowbars to stuff they cant get out.
ive seen it. and its not pretty. guy down the street with a 95' soft top stang got his rear top ripped out and subs stolen. im surprised his rims were still there. i bet they didnt have enuf time. (was at a concert or somthing)