So I've been looking at this more and more... and I think I have it, finally!
Side boxes running the length of the trailer. They would start at the top frame and run all the way down to the top portion of the fender. I'm thinking 4x6 angle might be a little too much for a box frame, though. What about heavy duty plywood boxes? Would be a little easier to cut out and put together. Alternatively, I can get some smaller angle to make the frame out of? Only reason I'm doubting the 4x6 as a frame is because it would be about 13' on each side of 3/8" thick 4x6 angle! Thats a lot of weight.
So I've been talking to neighbor (who has been my steel supplier for this whole project)... Looking at the following...
Option 1:
4x6" angle ran along the top of the trailer frame front to back. I would run the 6" length horizontally to get a deeper box, more suitable for shovel, ax, hilift... Then, take the sheet steel I have which is like 3/16" I think, and cut 2 8" strips lengthwise. Notch and bend to make like a 6" C Channel, then bend and weld that to form a 16"x63"x16" U shape. This would be the sides and bottom of the box. I would use the rear flanges to bolt and weld it to the trailer frame. Then I would just need to make doors. The remaining piece of sheet steel would be enough to make 2 16" tall doors that I would get some smaller tube to frame.
Option 2:
He's checking to see if he can get me some thinner walled C channel, 6" wide, which I would make the entire box frame out of. It would be strong enough to support the cap, lighter than using the 4x6 all around, and less work than notching and bending flanges in sheet steel. I would still make the doors out of sheet steel and smaller tube.
I'm leaning toward option 2, to be honest. Hopefully I can get some thinner C to use.