I took my clamp to the local hardware store yesterday. Not a big selection since it is metric as heikki mentioned - 6mm with a thread pitch of 1.00. Standard nomenclature would be M6x1.00, best fit was a socket head bolt since the outer rim of the head was knurled, not smooth. Even with that I'm not sure it can be tightened enough in the field without an Allen wrench to keep the insert from rotating. No thumbscrews at my local store but using a standard head bolt, a metric wingnut and a plain metric nut, I managed to fabricate a setscrew I wouldn't hesitate to use in the field. Total cost for the socket-head bolt 30cents, for the cobbled together set screw 90 cents. In both cases I found I preferred a bolt 25 mm in length. If there is a supply house near you that specializes in nuts and bolts, they will probably have a thumbscrew but for less than a dollar, your local hardware store should have something you can use.