Box trailers are useful for transporting goods and equipment that might slide off a flatbed trailer or need to be protected from the outside environment while being hauled. Any standard flatbed trailer can be converted into a box trailer in just a few steps.
Cut the Sides
Measure the frame of the flatbed trailer you're converting, and subtract 1.5 inches. This is the maximum lengthwise dimension for your trailer box. Cut four sheets of untreated plywood to the dimensions you need, and mark them near the edges for drilling. Put the first mark about 1 inch down from the top of each panel, and add another mark for every 4 inches down you can go.
Cut the Frame
Use an electric miter saw to cut sections of lumber into 2-inch by 2-inch by 96-inch braces for the box frame. Drill 3/16 pilot holes into the braces at the same intervals as the marks on the plywood panels. These holes may be left open, or you can guard them with metal studs prior to inserting the bolts. This is more work, but it protects the edges of the holes and reduces wear and tear over the life of the trailer.
Attach the Frame to the Panels
Bolt the wooden braces onto each panel, and begin assembling the frame. The edges of each panel should meet without any gaps. If one panel overlaps another, trim away the excess plywood with a handheld jigsaw or other cutting tool. Bolt the panels into place, and seal the joins with automotive sealant. Cut two large sections of plywood to the dimensions of the box's interior area, and attach them to the frame as a top and bottom. For an open-top box, only one panel is necessary. Attach the floor panel to the side panels with drywall screws and glue.
Finish the Wood
After the sealant has set, run sandpaper over it to smooth its surface and clear away irregularities. Treat the wood with a weatherproof finish, and allow it to dry before painting. Paint the entire box with a coat of primer, then apply a topcoat of spray paint in the colors and patterns you like.
Mount to a Trailer Bed
Carry your box over to the flatbed trailer, and set it on top in the exact spot you want. With a pencil, mark the outline of the box on the flatbed, and measure distances for drilling pilot holes. Drive 3/16 wood screws through the pilot holes, and secure them with steel washers and heavy-duty steel nuts.
If your design has a lid, lay it over the top of the box, and install hinges along the front panel where the lip meets the lid. Cut a hole in the rear panel to install a lock, if desired. An automotive latch can also be mounted to the lid.
How to Buy Box Trailers on eBay
Search for the parts you need by typing "box trailer parts" into the search box located on every eBay page. If you need to narrow the results you get, use the filters on the left side of the page. You can also find the parts to build your own trailer by browsing through eBay's many category pages. Find a seller with generally positive feedback, and place your bid. Many items can be bought immediately through the Buy It Now feature.