Tria LetraJet Air Marker:
Two-part epoxy putty such as Magic Sculp or Apoxie Sculpt…though plumber’s putty would probably work as well and is easier to find.
An old dried-out Sharpie marker, or one you’re willing to sacrifice in the name of science. You need the bottom 1/2 inch or so of the cap:
The LetraJet body diameter is designed to firmly hold a Tria marker in place. You’ll need to bore this out with a Dremel tool, or with a hobby knife or file and a lot of patience. When a Sharpie marker will fit through the hole, the previously-cut cap piece, once epoxied in place, provides the exact fit for the new pen. Use a little hot-melt glue or 5 minute epoxy for a quick hold while aligning the parts, then build up and secure the cap with a more generous wad of epoxy putty. You may need to tweak the depth a bit so the nozzle is in line with the tip of the pen (see last photo for depth reference).
A little piece of Fun Foam glued under the body provides friction to help keep the pen in place.
Taa-daah! Sharpie airbrush:
Why? Because Sharpies are cheap and readily available. 30+ colors, including metallic silver. Adheres to a broader variety of surfaces than Tria, Copic or Prismacolor markers, no need to mess around with a blender marker, and much quicker to clean up than “real” airbrushing.
Seldom requires more than 30 PSI, so an inexpensive air compressor is adequate, or can use canned air for small projects.
It does dry out pens like nobody's business though. I’ll sometimes use several pens of the same color in rotation, so each has a little time to “recharge.”
I use a good respirator when doing this (and latex gloves if holding the piece being sprayed). Sharpies are AP certified non-toxic, but I don’t know where this rating stands with regard to particulates or carcinogens. It’s easy enough, so why take chances?
Can’t vouch for the long-term durability of this. Sharpies are neither archival nor lightfast, but then most costumes only see infrequent use anyway. Don’t expect this to be All Things to All People, just another tool in the arsenal.