Affordable solution to make bunks more comfortable?

We purchased a couple of really cheap (like $2/each) pool mattresses and put them under the mattresses in our bunk house. These are slept on by kids, so there is not as much weight like it would be for adults, but the kids said they were wonderful. We only put about half the amount of air in them as should have been, but enough that if they laid on them, it wouldn't go straight to the wood. They lasted an entire season. They were not long enough, but they didn't need their feet cushioned anyway.