First time I took my keg setup mobile I lost my nylon o-ring. I had to get some more CO2 on New Year's Eve (great timing)
Fortunately I found out that in a pinch you can carve an o-ring out of the plastic thread protector that the welding shop puts on the tank every time one is filled. The plastic is thin and very pliable, but it does work.
Had a great New Year's at a friends with no leaking CO2