I put 2 free-air Kicker 8" subs in the rear deck of an 85 Oldsmobile and they work great for my needs. You don't need much power when doing IB, I have less than 200 watts and they're plenty loud.
You need to find subs athat are "IB" capable (infinite baffle). The big deal is reinforcing the deck so it doesn't rattle & buzz and sealing the front to back.