We had one built for our 109 so we could take part in the London-Sydney air race in 2001. I can't remember how much it held, but it was a steel tank on the back floor, with a tube that ran through a hole in the floor and fed into the left fuel tank via an electric pump. But the 109's AUW limits stopped us from carrying more than full fuel and a toothbrush. Each. Bit rough for 3 weeks away.
it wasn't very successful, the pump fed slower than the engines used it, so the transfer had to be on almost from engine start, but if we turned it on during taxi it overfilled the tank and vented out. Crossfeed was used judiciously to keep both tanks even. The aux tank emptied just before the mains did.
We elected not to take part in the race, ( too many opportunities for things to go wrong, and too many off-race refuels needed) and the tank is probably still sitting in the engineer's hangar in Bankstown, Sydney.
