I was going to do just that.......I found a cheap cover so i never did.I was going to get some ext. grade plywood and make it in two sections. Over hang the wood on top of the box rail a 2 inches, Use 2x2s to make a inner frame that fits under plywood up againgst the inside of the box rail. Attach front/against cab area with some hinges/or not.Support the plywood with 2x2s underneath for support. You will need 2 or 1 and 1/2 sheets.