Well, I am have pretty much decided against the enclosed trailer. I am have been playing around with an idea for a wooden box. I just hate to build an ugly plywood box. So I was toying around with some ideas.
I am thinking of just buying construction lumber (pine). Plane it down square and building a frame and panel style box. Making the panels out of exterior grade plywood.
I would probably buy 2x10's and rip them down since you get a much better looking piece of lumber than with 2x4's. Mount the plywood in the panels bedding them in caulking. That would help keep the water out of the joints. Then stain/point the whole box.
I also thought about putting a lattice instead of panels. Since this is not going to be water tight it would allow air flow to dry out wet 'stuff'. I could use the vinyl lattice.
I would add a PT plywood floor and add some cleats to hold it in place. The brace across the box would line up with the trailer so I could attach it to the existing metal work.
The center brace would allow for a place to attach hinges. I was thinking two piece top so it could be opened from either end. I could add a hasp and for lock if needed
Here is the rack. I drew it just using 2x4's. I was thinking of notching the uprights so that there is no chance of it racking. I could then just bolt it in place. Cheap and easy.
Same with the cross bars. Ship lap joints for rigidity. Put a couple of stainless or galvanized bolts through the joint with some caulking to keep the water out and it should make a long last rack. Even if not very elegant.