If you have common woodworking tools such as a chisel, hammer, clamps, utility knife, stapler and an electric saw and drill, you can build a good looking box trailer with a kid's seat and 100 pound load capacity. The wheels are scavenged from a kid's bike, but you'll have to buy eight lengths of 1-by-2-inch wood for the frame, 1/4-inch plywood for the bed and 3/4-inch plywood for the side walls. You'll also need carriage and axle bolts, screws, glue, nails and staples to fasten everything together. Axle bolts connect the wheels to opposite sides of the trailer, and the hitch is a strip of old tire sidewall wrapped around the seat post and bolted to the trailer. For complete plans for this homemade cart, see Reference 1.
El Cheapo Trailer
You can build this trailer for as little as $34 so it really lives up to its name. It's a large flatbed design that carries a lot of baggage and won't flip over when you turn a corner. You'll need 1-by-2-inch lumber for the frame, 1/4-inch plywood for the bed and some basic tools to make it happen. Use 90-degree brackets to screw the one-by-twos together and electrical box cover plates to make the wheel support braces. You can scavenge the two 16-inch wheels from a kid's bike and use an old car bike rack for the tow arm. Make sure your loads stay put by attaching eye screws for bungee cords to both sides of the flatbed. For photos and more information, see Reference 2.
DIY Bicycle Trailer
This fully enclosed box trailer uses 1/4-inch plywood for the top, 1/2-inch plywood for the sides and back, and 3/4-inch plywood for the front and bottom. Scavenged wheels from a kid's bike are attached to 1/8-inch steel plates on the sides, and a wrought iron tow bar is bolted to the front of the box and fastened to the seat post. You can build a 1-by-1-inch frame to support the top and put "theater" hinges underneath to make it easy to remove. Carriage bolts, wood screws and glue hold everything together, and common carpentry tools like an electric drill, jigsaw, vise and crescent wrench are all you need to do the work. To improve visibility, you can add some battery-operated taillights to the back. For more details, see Reference 3.