skep. if i were you. get a pair of 15's and mount them facing the back seats with a port in between them. then use the top as an amprack (flushmout of course) with a 'covering' of like 1\4" pressboard with carpet on top to hide the amps durring the day\night.
then make a 1" baffle thats slanted a bit to flush to the FLOOR of the car up to just below the windowline on the baffle - dont forget to put the port in and cut the speaker rings -, then a parallel (to the ground) 1" board from the top of the baffle to the rear deck. and then some 1" side pieces, and glass it all together (from inside and outside if possible) and seal it to the car with xpandifoam.
instant wall. well.. not instant. but quite **** fast... and should have like 80" deep down the center at least.
take a picture of the iniside of the car (few different angles) and the trunk and show us how it looks. i might be able to photoshop it and show you what i mean.