Just a thought,
My wife gets a little plastic bonnet thing at the supermarket to cover left over food dishes in the refridgerator. They come in all sizes. I wonder if something like that would be of use to the guys in serious weather. It has a elastic band in the skirt of it so it will conform to just about any shape.
The custard dish size is about the same size as a fuel cap in diameter. Would something like that help? It is disposable, cheap, conformal, readially available. The plastic might not be able to take sever cold, however we put them in the freezer without problems.
Just lock the cap, tighten it securly, put the plastic cover over the cap, and it should be pretty well protected from water getting into the lock area.
Just a thought,
Bob Weis