I use a Carry-Freedom Yframe large (20"). I strap a plastic bin to the flatbed and fill that with stuff. I have carried bricks, topsoil, building waste. There is a limit to how much you can carry in a brakeless trailer and one binload is my limit. The bin/flatbed IS my wheeled container.
For large loads, I can strap directly to the flatbed. I use a variety of hacks to keep long things at the correct angle to prevent them striking bike or ground.