If you don't mind me asking, where the custom cushions expensive?
Are you expecting, or experiencing much rattling from the loose hatch covers and fold over section?
I'm thinking back road wash board type roads.
I'm have something similar, although mine is a bit shorter. I gauged it on sitting on the bed head room. I got a $200 IKEA foam mattress for mine. Has been great, except when you have to move it, it doesn't fold real well.
Do your bins underneath move around and make much noise? I am thinking of doing something similar in the bed of my truck.