Stop in for a cup of coffee

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Never made any clamps like this for welding :rolleyes:
Use a chunk of 1/2" plate for the base section that gets welded to the work, on thin material it will soak up a lot of heat.
Just a couple small tack welds and you can exert a lot of pressure and then just break it off after the work is completed.

PS put the tack welds on the base plate closest to the pressure point. Leave the away sides unwelded for easy removal. The down force will keep those off sides in place