I had this same problem when I first got my keg. 5/16th gas hose to a 1/4 disconnect. I clamped the hell out of it and it fit on the 1/4 inch. It sealed pretty tight but looked a bit loose, so I came across a 5/16th to 1/4 splicer and used that with no problems.
Teflon sounds like an idea, If you clamp it down hard enough should do the trick.