Lee,
I realize this is a bit late, but I just saw the thread :/
I made my own a while back, basically read the following DIY and did something very similar, except I wired an additional outlet for my plasma cutter. Using it for a Miller 211 Autoset MIG right now without any problems at all, wiring didn't even get warm after welding some heavy duty D rings on a trailer.
DIY:
HOW TO - make a 220V extension cord - WeldingWeb™ - Welding forum for pros and enthusiasts
which I got through this thread on a search:
220v Extension Cord - WeldingWeb™ - Welding forum for pros and enthusiasts
and mine:
