I've found out that a slightly over-sized brass tube fits perfectly and air-tight in the smaller (4mm) Festo fittings. I had a "standard" DUbro tank set up and have always up-sized the brass tubing for them using some K&S stuff from the hobby shop (will measure it later). The 4mmx6mm Festo went right on and pressure tested perfectly. Then you can use a standard stopper set-up. Not nearly as elegant and clean, but keeps me from having to flare the tubing and use zip ties. Easier to unhook as well.

I'll take a picture this evening.