Summary

Basic screwdriver-type handle with a quadrant driver.

Instructions

[Note: I ended up printing it by flipping the model vertically and sinking it 3.5mm down into the bed. There was a tiny bit of sagging at the top end of the handle but it printed fine without support. Neither of these changes currently exists in the uploaded model.]

"Quadrant drive" is what I call this simple system of transmitting torque between parts using alternating quadrants of a circle. (I've never seen this used before so if it exists and has a name, let me know!) There are a number of advantages:

Strong: 50% of the material from each mating part contributes to the torque. Useful when both materials are the same strength, as in 3D printing. Allows you to transmit lots of torque for weak materials (like 3D-printed plastic).
Universal: almost any two quadrant drives can connect. There is no male or female side, and the diameter is largely irrelevant. You could attach a 1" diameter shaft to a 10mm diameter shaft with no adapters, for instance. Or attach two short shafts with quadrant drive to each other to easily extend it. Put it on screw heads, gears, pulleys, sprockets, shafts, hand cranks, robot wheels, etc.
Positive: Unlike other drives, all the mating surfaces between two parts (like a shaft and a gear, etc) are exactly perpendicular to the axis of rotation. This means that the driving or driven part can't round off if it's pushed too hard, the way most other drives can (hex/allen, Phillips, etc). The failure mode is basically all or none: either the entire thing breaks (in which case you need a stronger material or larger diameter!), or the whole thing stays intact.
Won't cam out like Phillips or many other drives.

I first machined a quadrant drive out of aluminum and some other unknown soft metal when I needed to connect a pulley to a drive shaft. I wanted it to be as strong as possible and not to deform over time. It was easy to machine. I ended up also drilling a through-hole between the parts and tapping the shaft so I could hold them together with a screw, and it worked beautifully. This approach would be very useful for things like attaching robot wheels, for instance.

I'm planning on adding some other parts that use quadrant drive. Hopefully I'll get the time soon.

Quadrant Driver