I was originally inspired by zignig's Printable Desk Clamp found here. I redesigned base to be more stable. This was accomplished by modular build instead of singe piece. However Mrk II base designs was more successful for printing as it had more simpler shape. In addition the moment up and down of the clamp was resign to be more sable. This was achieved by a small wheel instead of a sphere. Finally the clap itself has the ability to be as wide as needed. This is done by increasing the length of the threaded rod and square brass tube.
You can download the STL files here
#10 Half-Nuts x13
1/4-20 Bolt 2in long
1/4 Wing Nut
Rods and Tube
130mm, 120mm, 80mm, 50mm of #10-24 threaded rod
120mm x 2 Square brass tubing 3.2 mm outer diamter. Rails
20mm x 6 Square brass tubing 3.9 mm outer diamter. Guides
(Test the tubes at the store. The smaller tube should slide easily and snugly into the larger tube.)
Cut six larger diameter pieces 20mm long that would serve as guide for the rail which is the smaller diameter brass tube. Then tap then in into the stopper and hands with hammer.
Also, if the threaded rod is difficult to insert into the the plastic parts then you can drill them with a 6mm or 3/16in drill bit.