Ok, this is kinda embarrassing: About a year ago I added knobs to go on screw heads. They got used by generators like the Rotary Attachment (a contraption to laser engrave round objects) or the Pole Hook (a contraptions to hook a huge recorder to a music stand). Turns out they were not available anywhere else - although they are pretty handy for all other kind of thing. So:

ConcaveKnob and WaivyKnob

As always probably not the best names, but at least they are reasonably good looking. Please actually measure the size of the hex head you want to use and may be even reduce the value by a tenth of a Millimetre or two. Hex heads are typically a bit smaller than their nominal width to fit comfortably in the wrench while you probably want to press it in.

Not so good looking is the text output as rendered on my server. Locally everything is fine but for a while now the Cairo library only renders weird characters instead of the size of the reference block. If anyone has an idea how to fix this (on libcairo on Python 2.7) let me know. Setting the font face does not help.