a little pricey, but NIIIIICE!!!!! measure your bed and get the one that would fit best.
they make ones that fit the bed perfectly... there has been a thread discussing it before. use that little searchie button up top and type in "mattress" and you'll find plenty.
You can have them custom cut a foam mattress for you bed size. Not as storable as an inflatable one, though. Something about the right size for the bed (I just now put in 42" x 72" x 4" looking for prices), ranges from $250 for the really good stuff to $90 for a "random" piece. I bought a random piece a while ago, and it is good enough for camping.