This is a very useful Vice style, self centering Clamp that is 3D printable on most RepRap printers out there. Recommended materials are Nylon or Carbon Fiber filament for stiffness and durability. Use M6 screws and nuts to mount it together and find some 3mm Diameter sticks to fit the ''hands'' to the ''arms'' (see names of components ''Main'' and ''Bras'' in the model for reference).
For our particular needs (PhotoVoltaic cell characterisation apparatus), I made those 8mm diameter, 3.1mm deep holes for some standard super magnets that will be super glued in place to hold a 0.7 thick steel sheet, custom cut, to be able to place micromanipulators on top of it with more super magnets. The 2x M6 screws and nuts half-holes between the 2 big parts (Body Top and Bottom) are 75mm apart to match labware metric standard, but in our case this is the attachment method to mount this clamp to the Gimbal of the apparatus once the PV sample has been clamped and micromanipulators adjusted under a microscope.
As I show in this model, the ''arms'' of the calmp can be taken out and inverted to be able to clamp smaller objects.
I included the SketchUp model so you can modify and adapt it to your specific needs. Note this clamp is not heavy duty as is, you migh want to design new arms for that...