i got a hatch ut i assume that you guys have a plastic cover underneath the carped that sits over the spare right? well i went to bunning, got some strips of metal, only thin, 5 strips, length of the sub box, just a little longer, like 5 cm so 2.5 on each side. made a box, drilled holes in the plastic and then screwed this to the plastic, than to this i attached the 5th strip which i cut in to 4 pieces and bent making a brace, which the sub sits in. its braced by the front at one point/ corner, and sides at three points/corners. the back sits against the seat. when i need to remove it i just drop the seat and it slides in to the cabin
alternatively either glue or velcro the carpet to the plastic, then velcro the sub to the carpet, or put eyebolts on the subbox, with a nut n washer inside the box, and likewise to on the seat. at least one on each side, and then get a bit of rope, or that mettal thinggy that has two screws with opposite threads and like a swivel joining them, so when you spin it it either loosens or titens it. this will stop it moving. will b ugly as fuk tho
LOL look how long this is and its such useless info