Agreed, large lathe chuck and/or faceplate. Most I have seen made(home) are box shaped using flange mount bearings for the mainshaft and using a braided ground cable rubbing on the mainshaft to connect the ground from the welder to the work. Now for the rotation some have used a chain drive reduction but you have the problem of when your working with a lopsided piece it comes overcenter and jumps due to a slack chain. Yes you could use a tensioner but it will still end up jumping some. Now if you had access to a gearbox that would be closer to ideal and a DC motor and controller or an AC with a VFD I think its called would give you speed changes with the spin of a knob. Something I must say if I havent already is do everything possible to prevent arcing through the bearings and drive, doing so will improve the life span of your tensioner.