Reference the Auxiliary tank, Did you have a particular brand in mind, and are you going to mount it in the spare wheel well? I am also considering this option, however I was under the impression that I would get one that was piggybacked onto the main tank, and filled via the existing filler cap (therefore retaining the feature you mentioned which is just a ' metal flap' in the line.) at the flick of a switch a pump would pump spare fuel into the main tank.

