[JesseK]

My first guess is that the GDC will say no, that a pre-formed tube is illegal. The 24"x24" is stipulated as a sheet by definition. Hence, you'd have to come up with a legal way to join opposite edges of the sheet in order to make a tube. You may argue that you can melt the edges together so why isn't a pre-formed tube allowed, but the GDC may fire back that melting things together is a form of illegal soldering/welding. It's best to make a holistic interpretation to the rules rather than a case-by-case basis.

Hence, the best legal way to make your tube is to drill holes, overlap the edges, and put bolts or zip ties through. Or you could bend the edges backwards creating a crease, then lock the two creases together -- though something tells me that the creases will crack.

It's very possible that my interpretations are completely off, so I too suggest asking the GDC yourself.