I used the following from Amazon "Classic Brands Cool Gel Memory Foam Replacement Sofa Mattress for Replacement Sofa Sleeper, Queen Size"
It's a 4.5" gel mattress that we placed on the base plywood platform. We would place the cushions below the makeshift bed for the night. It can be a pain, but like anything else, it becomes a process. It worked pretty well for us. I'm 6 feet tall and close to 240lb and had no issues sleeping on it.
I looked into a folding mattress, similar to what you recommend, that's another great option. Just make sure the size works.
Good luck...
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