This is a cool vise clamp that can be mounted on a standard tripod with a 1/4-20 UNC (most common upon tripods). 

Warning: I have printed it and it works, but not tested it to the extremes. This one was designed and built for holding a firearm, and the stresses and forces haven't been tested to see if it can withstand the kick of a weapon. I am not held reasonable for the use of this; thus, print at your own discretion. As a normal clamp though, I have tested that and it works great considering it's made of plastic with a few metal parts. 

I will be building a better model later once tested. 

Print Settings:

• Material: PLA (shouldn't matter too much)

• Perimeters: 4 (gives a ton of strength) 

• Infill: 25% (could most likely do more)

• Supports: No

That's what I printed with. 

You will need to clean up your holes to allow the parts to slide nicely. The rails should have an interference fit with one side of the base, however, that may not be required. If parts are moving a little rough, sand them down or drill slightly bigger holes. Have fun!!

Also, there is a set screw design in this. If you have a lathe it is super easy, if not it could be done with an angle grinder and a drill if you have two people. (This is the main part of the design I hate; however, the only use case for it is to pull the jaw back which could be done by hand. So not required.)