4mm Screwdriver Handle

Print the main handle part in glass fiber or carbon fiber reinforced nylon only - I used Polymaker Fiberon PA6-GF25. This print requires a very well tuned printer to handle the small cross sectional area and tight tolerances of the bit socket, and also requires very good layer adhesion. 3 perimeters and 0% infill recommended.

Increasing cooling may make the socket easier to print but will also make it weaker to the point where it may not be usable. ABS, PETG, PLA, etc may work for low torque but in my experience are not stiff enough to keep the bit from twisting inside the socket with heavy usage. 

To hold the bits in place, glue a 4mm diameter x 2mm thickness neodymium magnet into the bottom of the socket using cyanoacrylate glue.