Harder and Steenbeck plastic cap

Works well for me. I use it as a protector when the brush is dry docked.

Does not fit the H&S Ultra (which most needs a cap for backflushing)

It doesn't fit over the flanges around the Ultra's aircap which most needs a cap for backflushing because the cage-style aircap prevents you from pinching the needle tip. You don't get an airtight fit so backflushing with the cap sort of works but also sprays paint out from the cap and back along the body of the airbrush. It does fit over the flanges around the Infinity's aircap, but you can easily pinch the needle for backflushing with the Infinity so you don't need the cap unless you're nervous about touching the needle.