The Box Kite my first trailer is built on a free boat trailer.
Strip it down and check the springs. Also look closely at the metal for cracks and such. Mine did fine for a couple of years then I hit something really hard with the wheel and a weld broke 2 years ago I was crossing a road that had aver high center and the trailer went up the car went down and the tung went snap and not where the chain was but farther back fortunately the metal on the top held but it was dragging on the ground I was 200 feet from getting on a very fast highway in LA area.
so before you start your build make sure it does not need a little welding or reinforcement.
Mine had been in the ocean so it had contact with sea water, and had sat in a yard for 10-15 years.
I bought a new hub with bearings and all. Put on HF tires which have done very well for me. Mine was a "T" shape so I had to add plywood with 2x4's and metal to support the front corners. I was fortunate to find a free metal pallet which I cut up and used it under my front end and under the front boot.
Good luck
Ron