50'' is the inside of the bed, if you want waterproof it will have to hang over the outside of the bed on all 4 sides like the orange one g&r posted.
anything thin will telegraph seam and grain through, that's why they invented landau foam, you'd glue that down first.
plywood will be quick and easy, if i were me... i'd make it fiberglass
''hydraulic dooheekies'' ? gas shocks?