If you are going for light, you don't need a roller to make your own round tubing from flat stock... find some appropriate steel plate (old car hood or something, imports have thinner metal), find something round and sturdy (automotive exhaust pipe should work) and find the circumference, cut a piece the length you want with the other dimension being the circumference of the tube, wrap the metal around the exhaust pipe working your way around, a rubber mallet should help massage things. Once you get half way around fairly close, use a C clamp or two and clamp that half to the pipe then you should be able to work the rest easier. If I had to guess you won't be able to get the ends closer than `" from eachother at this point which should be fine, take your metal and put a hose clamp around it or squeeze it and put vise grips on each end at the seam, then weld that together. Then you can put your bolt tubes and outlet tubes on then cap the ends like you did on the one shown above. If after welding the tube isn't round, you should be able to manipulate it in a vise or on a flat hard surface with a rubber mallet or something similar.

Electrical conduit is usually thinner wall than exhaust pipe, other than that I can't think of anything in the size you are going for.