I bought this:
https://www.amazon.com/gp/product/B0...e?ie=UTF8&th=1. Although, looks like they aren't carrying it anymore as a Prime item. Probably any foam twin bed you could do this.
Bunks in our 2920BH were slightly smaller than a twin. So I pulled it out of it's "sack" and used an electric carving knife with a board as a straight edge and trimmed it. Put it back in it's "sack" and installed it in the RV. I only trimmed the width, length was close enough to fit without any issues.
Steve