If the adjustable jaws of your F-style or C-clamps don’t have a protective cover on their heads, I’ve found that the caps of soft drinks or bottled water can fit quite well. They’ll prevent the metal clamp head from marring the wood, and they’re actually quite durable for this application. Depending on your clamp size, they might even fit over the head with just friction pressure, as with the clamp shown here. Or, if they’re slightly oversized, a little silicone caulk or a drop of epoxy will help to hold them fast.
– Len Urban
Rancho Mirage, California