Not sure how small of a fire-extinguisher you want, but I grabbed a small thin household one from WalMart for $14 over the weekend, for the GF's place.
I have a spare one that I've always
meant to carry, just in case, but never have. Perhaps with a proper mounting solution, I finally will.
No roll bar here, but I'm going to see if I can get a friend to fab up something similar to this, to mount at the base of the passenger seat:
