After seeing this design 

https://www.printables.com/model/1377221-mini-bit-tool-v2-variable-bit-fitting-perfect-grip

I wanted to have one for 4mm bits, but It has been pretty hard to find one and the ones that I have found they didnt work as I wanted, soo after printing the design above I desided to 3D model from scratch a similar looking design but all made by me and that would also have the mechanism for graving the bit firmly.

Soo I started designing in onshape as a chalenge and after 30 minutes I came up with this, I used petg but PLA is also fine.

Print settings:

3 walls

100% infill

0.2 layer height

Inner outter inner wall order but this one is optional

Soo If you like this model and 3d printed it, pls upload a making foto and a comment, you can sell it and do whatever you want with this model, this model is for giving to the community.

Also there are also the step file if you find it easier to do remakes or add anything like a keychain.

Have a nice day :D