My trailer is 4.5' x 6' and I used 1" square tube and 20 gauge steel. It is a toneau style lid hinged at the front. No RTT or plans for one. I wanted dry and secure storage and as light of a lid as possible. The metal warped when it was plug welded. My friend/fabricator did what he could with it. To do it again, I actually would consider wood and epoxy resin or perhaps aluminum sheet over the frame and use a fastener rather than weld. I think he had a lot of time in it messing w the sheet. Much heavier gauge steel and my lid would have been a lot heavier. I forget the difference but there are calculators on the web - use them.