Agree with using foam mattress toppers. We still had the 3" (I think) kig-size toppers from our pop-up, so I trimmed one of those to full-queen size (for the master bed in the 5er), and cut the other into two separate toppers for the bunks.
Using an electric carving knife to trim the foam to size works amazingly well. Just mark the line on the foam using a sharpie, have the foam flat, or ideally slightly elevated (I used several lengths of 2x4 I had handy) above a flat hard floor, and just cut through in a slow steady motion.
Haven't found anything else that will cut through it as well as an electric carving knife. A jigsaw, or even bandsaw, won't work well because it just tries to push the foam out of the way. The double reciprocating blades on the carving knife make short work of it though.
Very useful if you can't get a custom size foam to fit those non-standard bunks