I built a trailer to pull behind a bike. There are bulky things that without borrowing a car are tricky to move; this should help:

The rope is to suspend milk crates so they are below the level of the axles and give a nice low center of gravity. I'm considering using bungee cords instead for some suspension, but I need to think about how to be sure they won't stretch enough to touch the ground. I still need to paint the frame and add brackets to attach bungee cords to for holding the load.

The frame is made from 2 and a bit 8 foot 2x4s and is 28.5 inches by 34 inches. Glued and screwed together. Asymetrical because the wheels have different widths. The wheels are the oddball 28 inch (635mm) 700B westwood roadster wheels that came with my flying pigeon. For dropouts I used galvanized steel "mending brackets" from home depot with the dropout slot cut with a drill press and hack saw.

I made two different hitches, one for my bike and one for Julia's. For both the hitch itself is a QI250 quick dissconnect ball joint. I patterned the hitch after the bikes-at-work trailer hitch design. On Julia's bike I used U-bolts to attach it to the frame:

On my bike I used radiator clamps because the tubing on my frame was bigger than the U-bolts I had and it needed to clear the coaster brake reaction arm:

Several parts of the trailer are stronger and heavier than they need to be (hitch L-bracket, 2x4s for the frame, big wheels). The towbar arm and hitch ball may be smaller than they should be. Not sure.