I bought a Harder and Steenbeck Ultra Airbrush. The nozzle has holes on the sides, therefore it is almost impossible to create a backflow for cleaning or mixing colors.

There is an H&S Cleaning cap (127990), Reinigungskappe in German, but it is kinda expensive and the shipping cost too high, so I made my own.

I used TPU for the snuggly fit.
 

If you like, buy me a good german beer :)