While they're not a true "thumbscrew", look at the 1/4x20 nylon socket head screws at Microfasteners.com. The head is big enough to turn by hand, you can also use a ball driver/allen wrench to really snug 'em down, and they've got both 2" and 3" lengths. They're also cheap... $3.50 plus shipping for 50 of the 2"ers. If you need to make them shorter, just use a cut off wheel to cut them to desired length.