I find you can't go wrong with a 2 litre plastic coke bottle. When I'm getting a bit low out in the back blocks of NSW Australia, that extra 2 litres can guarantee me nearly another 50kms touring range. Just dump the contents in my tank and the empty bottle in the nearest trash can. Not sure how legal it is but I wrap the bottle in a soft hand towel to prevent chaffing in my saddlebags and better insulate the bottle. Have had no problems with this type of temporary fuel storage even after a very hot day of touring.
