Here is the completed project (excepting bolts which were added to both front and rear flanges later. The wood could certainly be waterproofed and/or ...
This method of constructing solid, wooden sides for a small utility trailer uses a minimum of materials and requires only an hour or two for assembly. We found all these materials on hand in the shop. This concept may be adapted for trailers of other sizes or where other materials are available for use.
Step 1: Fabricating the Sides
Determine the desired height of the sides. We chose 16 inches simply because there was a supply of +/- 33 inch plywood on hand in 3/4 inch thickness. The boards are first ripped to this height (width), then cut to length. The front and rear boards run the full width of the trailer while the side boards butt up against these. This allows for more fore/aft strength and security.