Holding small pieces at odd angles has always been a problem for me. I am have made a proof of concept magnetic fitting for a magnetic dial indicator holder. While the one I made was on a 3D printer is very heat sensitive it does work. Just some food for thought one of you may find a better way to do it.
Depending upon the strength of the magnet (and its proximity to the weld) it can aggravate a TIG arc in my experience. Might not be such a problem for a quick tack though.
As Cj stated, magnets interfere with tig welding and I've had magnets interfere with mig welding. When I need extra hands, I use a "third hand" that I put together using a connecting rod from Caterpillar engine and pieces of 1/4 inch round stock. Nothing fancy but gives me that extra support.
That looks like a good solution. For me, the hardest thing is clamping something that is curved. Clamps want to slide off.
I had to make a jig for this job:
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