frontrunner did this auxillary fuel tank a while ago.
I have it in my MK7 but it has some disadvantages - 1st, the Spare wheel is lowered an inch or so - not so severe because of the long distance between the axle and the rear end of the rig. My spare does almost never have contact to the ground.
And 2nd - the refueling is more complicated. The you have to do it more slow than usual, the fuel keeps comming up. I have the combination for about 150T km and though refueled many times. I learned to live with that but in countries where there are full service gas stations its sometimes hard to convince the service crew that the tank is NOT full just because the hose has stopped. They are offten stunned of the amount that could be added if you do it carefully
Dont know If frontrunner are still doing them.
Tim