Good sides make good trailers. Flat trailers can haul many things, but for big jobs around the house or farm, trailers need sides to hold in dirt, leaves, lawn debris, firewood or other materials. It is fairly easy to add sides to a utility trailer. Most utility trailers are 4 feet by 8 feet, dimensions that jibe neatly with the materials needed to add sides. To haul light things that blow off, such as mulch, add a tarpaulin to cover the top.
Locate the metal brackets on the sides and ends of the utility trailer. There should be two on each side and end. They are 2-by-4-inch metal rectangles welded to the frame of the trailer. Make sure they are clean of mud or debris. A 2-by-4 board should slip easily into them.
Cut 2-by-4 posts for the sides, two for each side and two for each end, to the desired height of the trailer sides.They usually will be 24 inches, but add another foot to extend below the mounting brackets. Measure the distance between brackets and lay the posts out to those dimensions. Lay 8-foot long, 1-by-12 boards on the posts; make sure the posts and boards are square. Fasten the boards to the posts with the galvanised screws.
Test one side by fitting the posts into the brackets. If the posts do not fit readily, use a plane or wood shaper to trim the sides of the boards slightly so they fit. For permanent sides, drill holes through the metal frame and put screws into the 2-by-4s.
Set the completed side panels into the brackets on the trailer and secure with screws if desired. Add metal corner braces where side and front end panels meet; mount these with screws. Leave the back panel loose for easy removal to access the trailer bed.
An option is to use 4-by-8 foot panels of 3/4-inch pressure treated plywood instead of 1-by-12s. Cut one panel in half lengthwise for the sides; cut two pieces along the plywood's width for the ends.