I once did one of these in a Catalina 22. Instead of building a center structure with legs, I simply screwed a set of long 3/4"
cleats to the lower vertical face of the settees. Then I cut a piece of half-inch plywood to rest on the
cleats.
The stock cushions rearranged nicely to form a sleeping surface with the irregularities smoothed by a standard fabric mattress pad.