There's a site on-line that shows a company that makes the sides and corners any way you want. There is an earlier posting on thes forum that had the link, don't remember which one it was, maybe a search would work.
I have built a few beds, using 5" channels for main rails, set on 5/4" treated deck wood, U-bolt it down. This includes a few crossmembers to keep striaght as you build. Then take 3" channel for your cross-members. Then use more 5" channel for the outside edge of bed, miter the corners if you can, or even make little angled corners of 4 to 6" or so for neat look. the 3" channel will fit in the 5" outer channel with a little trimming. Then 2 x 6 tongue and groove treated lumber makes a nice deck. the wood will shrink as it dries. I mounted mine temporarily, let set over winter, and drew it up tight with clamps in spring, bolted it down tight. Have to fasten some to keep from warping if you do it that way. There is a deck screw with square drive, that you just drill a small hole thru wood and into metal cross parts, and then drive the screw in. Self tapping. Work real good. Trailer places should have those. ON outer edge, I used rectangular tubing to make my pockets, then ran a 5/16 or 3/8" x 2" or so strap down outside of those to tie and strap things to. Works great! I also built a head-ache rack at cab end, keeps stuff from sliding into cab if get's loose and from hitting when loading and not paying attention, Ha! Big U-bolts slipped over the channels and welded on, with head sticking up make great tie down loops for cars or big stuff. Use treated lumber if you go that way, reg stuff will rot out in 1 year, well painted or not. I never did find a paint that really stays on when you use the bed for gravel and dirt hauling. I used Horse fence paint from farm store. It's what they have for replacement of Creosoot?, which you can't buy anymore. The tongue and groove keeps dirt and sand in truck when the boards shrink, not letting it get by. Without the T and G, the boards shring enough to have a gap that will cause quite a load loss!
