Affordable solution to make bunks more comfortable?

Try a nearby upholsterer. They buy foam by the sheet and can cut and glue it to any size you want. When we buy some, they even come with zip off covers.

Lots of people with pop ups use a memory foam topper, and they say if makes a huge difference.

You could probably get a full, or queen and cut it in half.

You can buy egg crate foam toppers ($10-20 for queen size) and cut to fit. Again, over or under. And they won't deflate :).

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.

How about getting an air mattress (or a couple of those blow up recliner swim things you lay on when swimming), and put them UNDER your current mattress. At least, give it a try. If the cheap-o swim ones help, then maybe you can invest in a more suitable blow-up mattress, camping mattress. If you don't want to use your existing mattress, toss it out and get a nice Coleman blow up air mattress used for tent camping. Some of them are like, 18 inches thick! Some are only 4 inches ... those might work well UNDER your current mattress to make it more suitable. Just a thought?????