I added a 3" memory foam topper underneath
the standard Denver mattress (it works just like a box spring), then added a 1" corrugated foam topper on top. It's the most comfortable bed I've ever slept on! The bare foam stays put on the wood platform, and the deep fitted sheet covers the whole thing. I also put a heated mattress pad on top and it keeps me toasty warm on cold nights, while saving a ton of money on propane. The struts would no longer hold the bed up when it was fully made, so (following the advise of a friend on this site), I moved them forward 4" top & bottom. Now all is good!