I needed a handle for a locknut M3 bolt to easily Z level one of my printers and I designed this quickly to do that job. Works fine for me. Outside diameter for the handle is ~15mm for my coarse fingers (works fine also with normal fingers). Insert a M3 nut into the hole, then screw in the M3 bolt, then insert the second M3 nut in the M3 bolt from the downside of the base and tighten it, there is no hole for the M3 nut in the downside to allow an easy tightening. Press/insert the top part (optional) to close the thingy and you are done. Tolerances taken into account, it should fit well. Use some glue if you want. Dimensions 15-16mm diameter x 15mm height for the normal one and 15-16mm x 9mm height for the slim version. Prints without support. After printing some of them I recommend usin PLA better than ABS cause this is a small thing and you need resolution, otherwise ABS prints will require some sanding. Not a heavy duty part, you know, it is just a thumbscrew. FreeCAD source included just in case you want to customize it. This is an enhancement for my other design here: http://www.thingiverse.com/thing:1504679

Hint: You could Z-scale up it a bit, but remember that the M3 nut insert will also grow...
Hint2: You don't have to destroy the top cap to remove it, just unscrew the downside M3 nut 15mm, add a second m3 nut to make there a locknut, then unscrew the M3 bolt using the locknut, the cap will be pushed out by the screw's head.
Hint3: Don't force tightening the slim version, it is fragile.

Enjoy!

UPDATE (24-MAY-2016): Increased nut diameter to be able to insert nut without pressing/warming. Tolerances are better now.

UPDATE (27-APR-2016): Increased nut diameter to be able to insert the nut without having to press fit or heat it.
UPDATE (26-APR-2016): Added a slim version 9mm height.
UPDATE (25-APR-2016): Added a round top model of the handle as suggested by a friend.