If you find a picture from the factory of the new Plus 8 variant of the Aero Chassis (someone on here is bound to have one!) you will see that unlike other versions of the aero chassis it has circular cut-outs for the speakers in the tub forward/below the A-pillar - on the new 'Aero' Plus 8 the speakers are not mounted in the door. I wonder if mounting your speakers similarly to the new Plus 8 is a more permanent solution - albeit you would have to cut the apertures using measurements from a new +8 chassis as a guide.
Personally I wouldn't put the speakers behind the seats. On my Aero with a six-speaker set-up the rears don't really add much, the sound gets left behind - and that's with a metal roof!
