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.
I found a guy that sells that same tong on another forum for less than I could I could make it for.
I bought one and it works, which was a surprise to me.
But this tong really works; AK Hunter1 says it is hard copper tubing with heavy copper cable inside. And it has a MIG welder tip that is replaceable.
He sells extra tips too. Here is the link.