The Harder and Steenbeck Ultra Airbrush 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 the shipping cost ist too high, so I made my own.
I used TPU for the snuggly fit.
I have added a second cap which fits the Harder & Steenbeck Fineliner. It is derived from https://www.thingiverse.com/thing:5369895
The author marked this model as their own original creation. Imported from Thingiverse.