I have rigged out several trailers like this over the years and my favorite method is using 2x4 stakes for the vertical members. Then use 6" wide green treated boards in 5/4 thickness. These are most commonly seen for decking boards or making docks and the like. The advantage is they are less weight than 2x6's, and all four edges are pre-sanded with radiused corners. At the front of the trailer I use hardware that I get at Northern Equipment (formerly Northern Hydraulics). They are like interlocking fingers that allow the front and the sides to detach from each other without tools for removal/storage. At the rear, I use two 2x4's vertically, side by side with a space between them so I can slide in a rear gate vertically...either use it or don't depending on the project. You can space the boards with gaps between them to improve airflow. I use carriage bolts, but from inside out, so the smooth heads are on the inside of the trailer. That way no possibility of hanging up as I slide things into the trailer and out. I do use a couple coats of Thompson's Water Seal on the wood after construction for general protection. I have kept trailers equipped like this outside for a decade with no problems...but I generally keep my stuff inside. You can use a hole saw to drill through the boards to open the area where d-rings are located if you need access to tie-down points while the sides are on. I usually make the sides 36" high on the front and the first 8ft of the sides and then angling down for the remainder of the sides, which I make 18-24" high. That way I get more protection up front for stuff I carry. Hope this helps. I'm about to rig out my newest trailer with this same setup for use next spring. We've had great fun over the years doing the hayride thing with our young friends at church as well. Have a ball!
