I would be tempted to try a JB Weld repair if I didn't want to buy a new clamp. I've never tried this, but I will describe what I would do: wrap several turns of blue painters tape around the threads right next to the foot. The finished diameter of this wrap must be smaller than the screw end mushroom. With the foot really clean, run the screw all the way down to clamp it against the C clamp anvil. Now build it up as evenly as you can with the JB Weld, making a donut shape between the screw side of the foot and the blue painters tape. Only get the JB Weld in contact with the screw side of the foot. Leave the business end of the foot clean so it can apply the clamping pressure as usual. When the JB Weld is cured, you should be able to remove the painters tape and the hole left in the JB Weld molded donut will allow the foot to wobble and self-align (probably only a little bit), but the hole in the donut is too small for it to come off the screw mushroom. As already said, just depends on how desperate you are, and perhaps your original repair efforts will work and you can claim victory.