Boler bunk cushions
Yes, the top bunk uses 3” foam on top of a 3-piece hinged platform of 3/4” plywood. That brings the total thickness close to the 4” foam used in all the loose cushions. The cover goes over the whole thing.
Do you have the old plywood base, or do you have a template for the three pieces of plywood? It will not fold down correctly into a sofa if you use a single solid piece of plywood. The ends have to bend to fit the curves in the front corners. I might have a sketch with dimensions on my laptop. Let me know if you want me to look for it.
Tim, if you make the top thicker, you also have to change the dimensions of the two lower cushions because the top fits behind the large bench cushion when lowered. It would need to be 1” narrower and the banana cushion would need to be 1” wider. I would not recommend doing that.