If you want to build but are lacking enough engineering skills to do your own design, probably best to just copy some successful examples. Lots of "Bob" trailers on Amazon with good pictures of the details. Bob's generally have two smaller wheels in-line. Also can find good pix of early Jack Taylor trailers common in the 60's and 70's. Same for box trailers which generally have two larger wheels, one per side. If you have a choice, recommend a design that connects to the seat post instead of to the real axle as have had issues with slippage on those.

Don't remember seeing a one wheeled cargo trailer...

/K