Finding an existing "sorta OK" trailer and retrofitting is likely many times cheaper than starting from scrap.
Yes, if you have a ton of tools, lots of time, etc., maybe building is cheaper.
Maybe.

Great previous comments. Maybe make/modify existing dollies, etc., still likely cheaper in the long run.

Yes, I can "build" a bicycle, motorcycle or car, but starting with something close is usually cheaper and easier.
Maybe not perfect, but what value your time to reinvent the wheel?