It's not metal, but I use one of [link=http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXJ810&P=7]these.[/link] Fits in the space on my field tote, uses the same size cap as the gallon plastic fuel jugs, and won't rust.

I also like that it's a half-gallon as it helps me regulate my obsessive-compulsive flying behavior at the field