It looks like Columbia Overland makes a spare tire mount for a single jerry can.Land Rover Discovery II Spare Tire Jerry Can Holder | Columbia Overland

I'd think twice about putting too much additional weight on the back door though. There are lots of people who have ended up with sagging door hinges that prevented them from opening that rear door. If you're already putting the 2 spares on the roof, why not put the jerry cans there too? It's not like they're going to cause that much more drag than the spares...