Basically you would just slot the mandrel to align with the current circlip location to commiserate with bit engagement. Then make a U shaped spring steel clip slightly wider than the slot. With the clip in place you will have pinned the bit on two side using existing locking features. Removal...
Take a look at the way a 2010 Corvette shifter knob is secured. It is basically a U shaped clip like a staple for electrical cable. It would be a simple mandrel modification and would be very effective for your application.