I created this air assist for my Sculpfun S6 Pro but it should fit the S9 laser. It replaces the stock acrylic cylinder and fixes in place with the set screw(s).
I used a cheap aquarium air pump from Wal-Mart with both outputs streamed into out tube with the check valves (I got a cheap kit from Petsmart with additional connectors, check valves, etc). This uses cheap tubing from Wal-Mart as well.
The tip is an inflation needle, the same thing used to inflate footballs, etc. The tubing and needle are connected using shrink tubing. The needle is fixed to the printed part using CA glue.
The angle is set so it should hit directly at the 20mm distance from the work piece and the laser. You can customize the length and/or bend the needle, however you need it.
You should also angle the needle, tubing in whatever orientation works best for you setup.
I printed this in Yellow PETG (color is what I had loaded and a darker color might be better). I have noticed improved cutting and engraving after I started using this.