First thing you can do, is measure out where the baffle is going to mount to on the rear deck and the sides/rear supports of the seats, then cut a piece of MDF or birch to that size, then do your cutout for the sub and put it in place and do a test fit to make sure that you won't be losing the function of folding the seats back up. Then get some spray foam and start sealing off the trunk. Both sides where the strut mount top plates are, are huge gaps and it's a starting point. The underside of the rear deck, there's a lot of holes and whatnot up there, and you can either spray foam in the holes, or use gorilla tape or sound deadener and tape/deaden everything. That will be a starting point when it comes to tuning it for the output you want.