M_Boats: Cockpit bed

Hi fellow Monty owners, over the coming week-end I am planning on sailing on Lake Michigan around Milwaukee, and I plan on spending one night with my two sons on our M-17 docked at a marina. The weather is expected to be nice. My plan is to let the two of them sleep in the cabin, as they are quite comfortable there, and to make some arrangement for myself to sleep in the cockpit. For this I intend to buy a couple of plywood boards which I would lay across the cockbit benches, and to put a sleeping bag on top. Has anyone tried this? Does anyone have dimensions for the boards one needs? Other suggestions? Thanks, Andrei.