Bump!
I know you said it's been sitting in storage for a while, is it the same as the last photo? Lets get this finished for the summer!
I'll happily drive down to help out. My vision would be sheet metal for the walls, treated plywood (strong and easily replaced) for the floor, removeable tailgate (vertical or horizontal hinge?), bunch of tie-down loops, removeable rack bars for hauling lumber or a tent... But, it's your trailer.
Expanded metal (mesh) walls would be best for a utility trailer, but solid walls will be better at keeping the interior somewhat clean when offroading. Keep the mud out! I'm picturing waterproof totes stacked as necessary with firewood next to them, and a tent platform up on top offering a little shade and rain cover. Could even add loops on the exterior to tie down a tarp and give it a removeable roof (picture military truck bed covers) to really keep things neat. If you do want to use the trailer with a tent, I'd suggest adding leveling jack mounts to the rear corners. These side-wind versions might be perfect for that. https://smile.amazon.com/gp/product/B004QEM7J8/
I can't tell from the photos, can you fit a gerry can where the side mesh sections are? If not, I'd put gerry can mounts right behind the tail lights after the walls are skinned. The rear might be better anyhow for weight balance, remember that you want more weight to the front vs the back, but not too much.