Have you tried a place like Foam By Mail?
Browse Memory Foam Toppers for Beds | FoamFactory Inc.
You can get a topper from them custom cut to your dimensions.
We used them to custom make a foam bed for our drop down bunk in the front. Its more comfortable than the other beds in the camper. So much so that the wife might kick the kid out of the bunk and use it herself! LOL
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