Oh man this brings back some old school wheeling memories...
I used to make wood flatbeds with some success. They can end up heavy, which isn't necessarily bad as it gets you better front/rear bias. I came up with my design technique studying old wagons and buck boards.
What I did was get decent lumber, doug fir at least. Hardwood like oak would be best but it's even heavier and rather expensive. Paint it all ahead of time with oil based paint, it holds up to road spray alot better. Bedliner is good to but more expensive and it doesn't really last any better than the right paint.
I used 2x6 for the deck and 2x4s for the sides and end rails, which I built three thick with stake pockets and ran the side on end even with the bottom of the deck boards so they made a slight lip on the sides of the bed it you can visualize that. This works great for tying down loads with straps on trail rides since it gives you a lip to grab the edge of your cooler or whatever and keeps things from shifting around. First thing I would assemble the entire bed out of just wood and decent deck screws, with 2x4 as cross peices under the deck and side rails, then I built up the under frame of the bed to meet up with the body mounts securely. I usually used treated 4x4 and 2x4 depending on how high I wanted it etc. I then mached it up and drilled the body mount holes through the wood with one of those long 3/8 drill bits and intalled carraige bolts through from the top to the bottom. Give them a good whack with a hammer so they set well and don't spin out.
Now here's the important part...
You want this thing to be able to take a hit on a rock or a tree without falling apart. Add angle iron against one side of all your cross peices under the deck and attach every board with two carraige bolts a peice. Then bolt some 3.5" chanel iron to the outside of the side rails. Now it can take many hits, flops and remain happy for years to come.
It you want top really finish it off put some sheetmetal over the rear wheel well area of the underside of the deck and put your fuel filler in an ammo can.
With the price of lumber, steel and bolts it will not be especially cheap, at least a few hundred dollars, which you could probably find a decent bed for, but I did this with a chop saw and a drill way before I had a welder and it was cool and worked great for years. This bed made in 1998 is still in service on a different truck today. At this point I have all the tools and a welder and I want to make a steel tray back for my '80 longbed with hinge down sides like the aussies do. I miss having a flatbed yoder. The utility is great and I always loved the "what the heck is that thing" look you get in traffic.
I had to scan some ancient history photos... We made a few more beds the same way. Mine was the red '94 and we also made the one on the blue '85 that I'm parked on in the sideways pic. We made a couple more over those years that were bobbed off at the shackles and dove tailed. They looked cool but were less useful as a bed. You really find with a flatbed that you don't need to be bobbed since you can drag the whole back end over everything anyways and it doesn't do any damage. I added a photo of my buddy's '83 with the flatbed he made of plywood. That one was pretty cool and also light but it eventually fell apart when he did a wheel stand on the back of it while crawling up some nasty in Tillimook. Man that's back when 34" TSLs were just HUGE!
