if it were me, I would buy a piece of particle board or MDF and trim it down to fit exactly in the spot where the back rest of the old seat would sit
then I would install a couple off carriage bolts in the baord and drill holes through the body so I can mount it
lastly I would upholster it in a way it matches the rest of the car
maybe, just maybe cut out a few holes for some 12 inch subwoofers
(that's the way I had the backseat in my old opel
Dukeboy makes a good point though, you might get a better response in the racers section
