Well, not really aux, but maybe an extra 70 or 80 highway miles. I installed the 6 Monkeys jerry can (20 liter) holder on the spare tire holder. Nicely crafted piece of work by them, and a cool design. Its all made for Jeeps, so I had to do a bit of work with the grinder and drill to get bolt holes to align, etc. It's hinged at the top and the two halves lock at the bottom.
I also need to drill a third hole at around 135 degrees on the pivot as the holder hits the car at 180 degrees, and the tail gate won't drop with it at the 90 deg swing position as it hits the jerry can holder...small problems for a bit more peace of mind in a car that doesn't have great legs on long drives.