Renovating RV, how can I get replacement over-cab mattress?

Renovating an '88 Winnebago and want to replace the slide-out, over-cab mattress/bed. I cannot find anywhere that seems to sell replacement cab over mattresses with the slide-out section. The forums I have read mostly suggest making your own wooden panel to cover up the open overhead cab space and then purchasing an RV mattress or foam topper, but that would mean that the space above the cab would have to remain covered. Does anyone know of a company that sells slide-out over-cab mattresses pre-made? Would an RV refurbishing company be able to make one?