Oh, yes, excellent idea!
We used a metal futon frame, half for each of our "twin" beds in our egg, and 7" mattressing from IKEA's As Is department. Surprisingly, the futon frame halves exactly fit both bed areas. Paul found it in a scrap metal pile at his old job and almost didn't grab it! Thank goodness for cell phones. He called me and I was yelling, "Get it! Get it!"
You can see the mattress--it's covered with an elastic "sheet" made from Walmart's 100% cotton lightweight upholstery fabric--the other one is the same style but slightly smaller. The frames work in both spots, though they are identical.
To the left of the twin mattress, you can see a bit of the permanent seat--same foam as mattresses, same type cover--beneath it is a hinged lid to the "pontoon" storage area--that side has our electrical
panel, extension cords, etc.