A box trailer is a towable enclosed space sized to meet expected usage with no cabinets, drawers or racks, so making one at home is not an elaborate project. Before setting out on any practical aspect of the project, learn about the regulations governing homemade trailers in the municipality or county where the trailer is to be registered. Almost certainly a Department of Transport inspection and certification will be necessary, and the registration process might require that a notarised document accompany the application confirming that the fabricator is a certified welder.
Visit a trade show and a trailer dealership to learn the best overall size for your purposes. Make notes on construction methods and the measurements of box tubing used to make the chassis, the uprights, the hoops and lateral supports of box trailers. Note the locations and mounting hardware used to fix the axles and the suspension.
Draw out exact plans for your homemade box trailer on graph paper. Pay particular attention to the tongue, which must be long enough that the body of the trailer would miss the back end of the tow vehicle should it jackknife. The length of the tongue is also important in creating a balance between gross trailer weight and tongue weight; too much tongue weight can reduce the tow vehicle's steering and handling, while too little can cause trailer sway.
Buy parts that cannot be homemade such as the tongue coupler, electrical components and the axles, wheels and tires. Determine whether the projected use demands electric brakes before ordering the axles. Chose hubs with a common stud pattern so that wheels are easy to source, perhaps even from a salvage yard.
Use a cut-off wheel or angle grinder to make the chassis rails, then weld the base together on a flat surface, ensuring it is exactly square. Turn the base upside down and attach the axles using "U"-bolts or welding on mounts. Ensure the axles are exactly at right angles to the side rails of the chassis and located so the unladen trailer is balanced, meaning that the tongue jack and wheels support it approximately level.
Fabricate and assemble the tongue, loading ramps, support hoops and lateral rails. Turn the chassis right-side up and make the door frame as sturdy as possible without causing a weight imbalance toward the rear. Panel the walls, roof, floor and door with the lightest materials that will give the desired strength, wind resistance and theft deterrence. Install "D"-rings for hanger straps. Install an electrical system that allows for at least one interior light to be switched on from close to the door, and another that operates the legally mandated chassis systems for the trailer's size and weight.
Jack up the box trailer and fit the wheels and tongue jack. Attach fenders large enough to prevent thrown rocks from being a hazard to the tow vehicle or other road users. Attach a spare wheel mount and spare wheel.