It isn't much work just to cut that square and cut a hole in it and then screw it against the back of the seat. Just do it provisionally and see how you like it. If you do you can make it look peeeerty afterwards! What sub would you be using?
Mobridge DA2 > JBL MS-8 > ARC XDi 600/4 > JBL Gti 408 Mids & Vifa OT19 Tweeters; PG Ti2 1600.5 > Morel HCW-10 & IDMax 15