Custom stop collars for forstner bits, as I needed some precise depth holes in the middle of a large piece of plywood which did not fit under my drill press.

I printed them in PLA because that is what was in the printer, and it started to melt after half a dozen holes, so I recommend something a bit tougher like ABS or PETg. Also go slow and gentle; my bit was dull so I had to press too hard and that did not help, but these got the job done. 

Included are 3 different sizes, as both my sets of bits seems to have 3 stem sizes, 10, 12, and 15mm. STEP file included if you need to adjust the fit or sizes. 

I also used modifiers (pictured) to increase the infill around the screw and nut, which seemed to help them clamp tightly without crushing.