dax has one available on his site. i bought it so here is a report;
The tank itself is of similar quality to the ones that ship with the kits, the gas plug is lower profile and leaks just like a regular kit gas tank. The risers that go down to the axles are heavy duty, but the mounting hardware leaves something to be desired. It is supposed to mount to each side of your rear fork up near the seat. I have seen bikes with places in the frame drilled for rear racks of this kind, but not on newer mountain bikes as far as i know. I has to improvise with some brake cable tightened around the seat post. It is designed for wheels less then 26 inches too, because i had to fabricate risers of a couple inches to clear my rear tire. I'm going to have to make more risers too to make the tank tilt forward toward the valve, because right now it's slightly tilted back.
It comes with some issues, but if you don't mind some work it is a great solution. Far stealthier then tank on the front. There are some home made solutions on this site, the plastic seat post mounted racks can be drilled to mount the stock gas tank to it, but i'd be worried about plastic fatigue in the long run. I jury rigged a rear mounted stock tank to a standard aluminum bike rack but over 4 months it broke from metal fatigue.
In the end,