Here is my solution. There is a Grainger close to my house so I go there, but you can probably find the same connectors at McMaster Carr. I choose the 1/8 NMPT thread so that I don't have to alter the black plastic cartridge holder. I drill and tap the end of a spent cartridge then use some 5 minute epoxy when threading in a coupler to create a seal. Since the wall of the cartridge is so thin plumbers tape won't make a good seal. I like the epoxy rather than a weld because I can still take it apart should I choose to change ends.
1/4 air chuck (air tool) end.
1/4 MFL fitting end (same thing as the fitting for my cornie disconnects).
1/4 Poly barb. I also have a 1/4 brass barb but no pic handy.