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 > JL Audio HD600/4 > (3) Hybrid Audio L3SE & (2) Rainbow Cal25; JL Audio HD900/5 > (2) Morel HCW-10 & (1) AE SBP15