Remove the Hatch cover, and build up a false floor to frame up on each corner/not completely sealed with 1/2"-3/4 plywood or mdf and carpet the top..Easy to do, and will give you better bass response/output, and sound performance, as well as room for the sub to operate properly, and a sturdy platform for your dog to lay on or placing gear on top..Hinged to still gain access to your spare and or sub maintenance if need be
You build on top of it/an addition just the height of the 2x4 framing..You hinge it as it is now to gain access to your spare tire..Should be as a permanent type floor, though can be easily removed to go back stock if to ever sale the vehicle, by removing FRAMED/False floor,and placing the old trap door back in stock location.VERY EASILY DONE..Ive done several vehicles this way over the years..Acura Integra(92&93)(90) 240 sx, Maxima(99) Honda Accord(2000) ,CRV(99),and others,and is very inexpensive to do..Maybe $60-100 total for materials, and about 4 hours of your timethat is a good idea, but the hatch on the wrx is very small and i dont think id be able to get a full size false floor in/out if needed all the time. the floor panels in the back come in 3 pieces. Maybe i could someow do it with 2 pieces?
LOL..You may need to sleep on it..Its very easy...Just take some card board and cut it out to fit about 1/2" gap on each side for a template for your Plywood false floor base..Then remove the trap door..Get/cut a 2x4 about 4" shorter than the full length of the floor from back seat to rear of hatch floor giving you some opening space underneath..lay flat..screw down to existing floor..two screws at top and two at bottom, one 2x4 each side..take your plywood/mdf and trace the cardboard to cut it out..get some spray adheasive glue and place your new carpet on top of tht(find at home depot for like $20 bucks for a roll of approx 4-5Ft x 60"/grey,black,tan.)and place it down and you can smooth it out by hand or putty knife,or? wrap it and cut off excess..Screw down on top of 2x4s..Doneright i understand that i build on top of what is already there, but what if i need to take the floor out to do something in the back of the car. i think it would be difficult as the floor is wider than the hatch in certain spots but maybe im just tired lol