Mine is 3/4" ply (Home Depot) supported by a 2"x4" frame (frame is screwed in with the 2x4s sitting vertically - not horizontally - for structural support). The framework rests on two side support shelves (lips) the width of the frame, which are thru-bolted to the sides of the benches - one on each side. If you span the on top of the benches, from wall to wall, then you won't be able to open the bench lids unless you modify them. Keeping the platform inside (between exteriors of both benches) allows you to open the lids on the benches easily without mods to the benches.The exposed front of the frame is covered with a piece of finished hardwood (1"x5") and matches the look of the bed rail, up top. The top of the platform is covered with indoor/outdoor carpet that also matches the FWC side bench carpet (also Home Depot). This settee bench top plywood is hinged in the back with a piano hinge (to the 2x4 frame) so it can be lifted easily to access the storage underneath. I also have an insert to make the bench wider for two people, but typically we use the this platform lid as a settee bench for sitting. It is wide enough for one person to sleep stealth without the insert. It easily supports two people with no sagging in the middle and requires no center leg support (although it does have a center 2x4 support built into the frame). My link below shows this as well. It's an easy project and relatively inexpensive.
Rich