This is a set of R8 Collet Blocks for various machine tasks I included both a square and hex shaped block. They apparently don't sell these so I made my own.

***To get these to work well you will need a 7/16ths" - 20 TPI bolt and a fender washer or two.***

The washer will spread out the clamping pressure and keep from denting the bottom of the block or losing grip over time. The fit on these is pretty tight to allow the maximum amount of travel and torque control. I would recommend a little light oil inside to help making installation and removal easier. To remove a collet just thread the bolt into the back and use a vise or a clamp to help push it out. If you want them so the collets drop in easier, increase the print size by .25%, but you will sacrifice some clamping pressure. A good tap on the ground with the bolt sticking out of the bottom will free them up. I added some print modifiers so independent of your print settings the bottom of the collet well to the base of the model print solid this will keep this space from compressing and allow you to generate maximum clamping pressure on the collet. Use at least 4 walls. The more walls the better, but it will work fine with 4. If you plan on keeping them for a while and not having them be disposable I'd print them with maybe 10 or 15 walls.