Since I could not find it anywhere and have already lost the original part twice, I designed the crank arm cap myself. A replacement (not even an original) costs around € 8,- on Amazon.
If you do not have the tool to screw it in, you can carefully press it into place after screwing it in a bit with your fingers…
Would very much appreciate, if you'd post your make! :)
I printed it as follows:
Only for design purposes, does not fullfil the specs of the original part!
The author marked this model as their own original creation.