Raising passenger seat?

I recently purchased SOund Ordinance's flat pannel 8" sub which I thought I'd be able to slide under my passenger seat. Unfortunately I need another 1"1/2 - 2" of space for it to fit. I can't find anywhere else to put the sub. I sleep in the back of my car (car camp) on a regular basis by folding down the seats and throwing a matress in the back and using a board to extend the floor to cover the gap between the rear seat backs when folded down and the front seats (BTW: if you haven't tried this you can almost get a queen sized matress in this space), so anywhere the sub would protrude and keep my matress from laying flat would be a problem. I also use the spaces behind driver/passenger for all my gear when I car camp... So is it possible to raise the passenger seat an inch or two to allow the sub to slide in? If so anyone have recommendations on how would I go about *safely* doing this?