Flat bed not too hard. 5" channel for two main runners, mount on 1" wood decking, cut down middle and drill holes for each rivet to set in. Use 4" channel all around outside, miter if you can at corners. Use 3" channel for cross braces if using wood for bed, or use more 4" to make level for sheet steel. Use rect tube for stake pockets, and run a wide 3/8" thick or so flat stock on outside of those for tie downs for straps. Old U=bolts with each length down opposite sides of a cross'channel makes good tie downs. OR, put them over the main 5" rails and stick above floor. Angle the bed after the frame ends, by cutting the metals at proper angle. Some needs cut off both mating ends, or it won't line up correct to weld if you cut just one. Some ideas I have done. For wood I like 2 x 6 tongue and grooove, or random size 2 x lumber and cut the dado in each edge. This is so dirt and gravel won't fall thru when hauling, as wood always shrinks if you use treated. Non=treated will rot in year or two. Never found a good paint or finish to stay on in weather or against wear. Best I used was Farm Horse Pen Paint, black. Kind of the modern Creosote replacement.
