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The shorter MOT is a very good idea; indeed you see it on the actual machine. however i would avoid the bearing with wrapped copper; once i needed to have extra long arms (+/- 50 cm) on a spot welder to reach the middle of a sheet and the only thing l had at hand was copper tubes and steel rod. the copper tubes were barely useful, there wasn't enough material to transmit the courant but the steel was truly useless. l would advise to use thick solid copper as much as possible. also as you said the trick might be to have thin edges on the wheel
Adding a small motor to have a nice continuous seem is actually a great idea