I haven't yet but I want to build a hand dump-able flat bed for my '46 ford sedan (its a long story[ddd) I am planning to build a frame and skin it with 16g sheet steel. Would be interested to see if someone else had done any bed work.
I have built a bunch of flat beds, and even the bed on my truck, but not a cover. It should not be too difficult. A light weight frame, and skin it with sheet.