This is the air assist nozzle I use with my K40 laser cutter. The design was created in OpenSCAD and I have included the OpenSCAD file that can be modified. The coiled hose and fitting I used are listed below. The fitting that I used to attach the hose to the nozzle came with a barb on the fitting. To use this fitting with the hose I selected the barb needs to be cut off with a utility knife. This is not difficult to do because the fitting is plastic.

The little plastic tabs that were intended to snap the nozzle to the laser head have broken off of my nozzle but it still stays on the head for me. I may update the design to remove the tabs or eliminate them entirely in the future.

I used a 1/8 NPT pipe thread tap (available at a good hardware store.) to create threads in the the plastic for the fitting.

Self-Retracting Polyurethane Tubing
Abrasion Resistant, Opaque Yellow, 5/32" ID, 10 Feet Long
McMaster Part: 9148T122

Super-Flow Barbed Fitting for Food & Beverage
Plastic Adapter, for 5/32" Tube ID x 1/8 NPT Male
McMaster Part: 2808K23