Inspired by the traditional f-clamps, this is a fully 3d printable version. You can use it to hold parts together after gluing them together for example.

Print instructions
Use 6 or more walls and 25% infill for the central bar and screw. Use 4 walls and your normal infill percentage for the top and bottom clamp. Other parts can be printed with defaults.

You want it to be as rigid as possible (especially the central bar) so I recommend PLA over PETG because of the higher rigidity of PLA. 

The TopCap and the ScrewTopCap can be optionally printed in TPU. If you don't want that you could also leave out the ScrewTopCap entirely.

I have added two different lengths for the bar. With the 16cm version you can clamp objects up to about 11cm. And with the 22cm version up to 17cm. I could add other lengths if desired.

A plastic clamp will never be as rigid as a metal version and this clamp will bend quite a bit when put under enough force. I find it good enough though to hold some 3d printed parts together for gluing.

Assembly
Assembly is simple and should speak for itself. You don't need any additional tools, though you might want to add some glue to the lid of the screwknob to prevent it from coming loose.

The screw thread may be a bit tight the first time, but after screwing it fully in and out one or two time it will run better.

If you like the wireframe fidgetcubes, you can find them here.