I recently got the SainSmart Genmitsu Jinsoku LC-40 10W FAC laser and couldn't find any air assist nozzles that fit it perfectly. Some of the xTool 10W air assist nozzles would fit around the lens, but not with the other tolerances that the LC-40 brought with it.

The design is made to go out the back of the LC-40 and might require you to remove some of the metal shieldings that come pre-installed on the 10W FAC laser module. I removed the baffling from mine as I no longer need it with using proper air assist, though you determine if you want to remove it on your own.

I printed mine with PETG at .2mm layer height and in the normal orientation with full supports and it came out perfect. I'm still making tweaks to the design, so if something doesn't fit right be sure to come back and check for updated files (Everything fits properly for me, but always tuning it to be slightly better.)