I added a 1/4 inch square hole in the back of the tool, to be able to use a ratchet and get more leverage when undoing a stuck pre-tensioning cap. You don't need extra leverage to tighten the cap, since it only requires 0.7 - 1.5Nm of torque to pre-tension the bearings.
Das Filament PETG
I recommend no less than 3 perimeters.
The 1/4 inch square hole is oversized by 0.4mm (about 16 thou), to have a 1/4 in drive shaft to be able to fit in. On my printer this gave me a pretty snug fit, but I had a tad of over-extrusion.
If you print tools and constructive parts a lot, you probably know the tolerances of your printer and material very well and I suggest changing the oversize to fit your printer and material.
Designing this thing was simple: I just took the STL for the tool from V4N, loaded it in OpenSCAD, and subtracted a 1/4 square hole. I didn't add a hole spring loaded ball retainer though. Not, that it would be needed ;)