I tried using tubing, 1/2 inch soft copper from the hardware store was too springy. I bought a same length of 3/8 inch and slipped it inside. It works well. I reused the tip of the HF spot welder, but if I were to do it again I think I'd use MIG tips.
Better would be a length of Type M hard copper tubing, if you have a plumbing supply in your town they should be able to fix you up.