I have a aftermarket swing tire carrier, also has a 4 container Rotopax carrier attached to it. Plenty of mounting options for a 2.5lb extinguisher (already have)
What thoughts of it sitting out in the elements?

Jeep is garaged
Not a DD
Do take it on 3000 mile trips once a year

If mounted, it'd be on driver side, between the carrier and swing gate

 

I'm going to do one on the tailgate as well. I'm going to draw up some plans and probably make the mount myself.

 

It depends on the quality/type of fire extinguisher. Extinguishers with metal valves won't leak , unlike plastic valves that has a tendency to leak due to temperature changes.

 

I would be semi worried about theft. You can't lock it or make it more then a single step to unfasten or else thats a big problem in an emergency when someones adrenaline is running and they are fumbling with some attachment device you thought would work well.

I mount mine on the sports bar so its only open to the elements in the summer. Had it stolen while parking in downtown along with some other stuff. Found a homeless guy wearing my windbreaker and fleece that was stolen at the same time about a block away. Got that back but never did find the fire extinguisher.