You can use just about any gas can securely fastened. I have used a Wal-Mart one gallon under a bungee net. Up to Alaska that is all James and Colleen did.
I made a 'cradle' for a one gallon Wal-Mart and sat it on the back of a TT pannier.
I've also used the 3L TT one, it's very good...stayed on the bike at a 60mph cow hit/flip!
Extra gas is nice. Even a one gallon milk jug works in a push.
For more rugged and well fixed I'd also Roto-Pak or a well built cradle for a one gallon jug.
MSR fuel containers work well too. TT has a mount for those.
NEVER pass a gas station....even if you are half full. I passed one at 3/4 full and still ran out of gas before the next one. How could that happen? Washignton State...Route 17 north...Fill with gas, head up past Ephrata, take 2 right at Coulee City...ignore gas station...take 21 north at Wlibur...ignore gas station....take ferry across...take 21 north towards Republic...run out of gas 11 miles short of Republic. I'm not even sure if there was gas at Wilbur, but there certainly is none on that stretch of 21!