My first design used a CD4069UBE in DIP package, which I had laying around at home, built on a perfboard. It seems to work well now, but I will still test it more.
I designed an other one in EasyEDA, using a much smaller IC, the NC7WZU04P6X. This version fits on a 6.6 x 9.2 mm (0.26 x 0.36 inch) board, that costs 0.45$/3pcs at OSH Park. I'm currently waiting for the PCBs, so as soon as I get them, I will test their reliability, and publish some more photos.
This circuit should work with a 5V supply. If you would like to use it with 3.3V, I recommend changing the diode to a schottky with a lower forward voltage.
Also, I appreciate all kinds of feedback. And if there's a need for it, I might sell some on Tindie.