Or here under LCF.

I'm not sure you really need plans... plenty of trailers on the internet to get some ideas, then throw together something that works for your needs.

What are your building skills? Materials?

What weight range are you considering? Style? What to carry?

For general cycling, it is hard to beat the small kid's trailers and small cargo trailers.

Personally I prefer to support axles on both sides, but single sided wheels are also popular, and becoming more common with the kid's trailers.