Just a quick and dirty extension for 4mm screw bits. Spaces to glue magnets if you're into that sort of thing. Print at 100% infill for strength. It's a quick print even at 100% and only uses a couple grams of filament. 

I have included a version with tighter tolerances and one with looser tolerances. Pick the one that matches your preferences and printer abilities. Don't like either of the version I uploaded? Make your own! The model is parametric. You can change it to fit any bit as long as it's a regular polygon

https://cad.onshape.com/documents/e65ca32281e1d0042e6f0890/w/ac91d904117933919256dd6a/e/f615f81351ec3674ddc4eb95?renderMode=0&uiState=68edbbb1b73f6730c5819099