You could always do what the late Ian Palmer did.
Get 2 tanks, cut the bottom out of 1 and the top out of the other, weld them together and you end up with a 5 gallon tank. (the Indian tanks would be suitable for this)
Just so happens I have an Indian tank on the shelf, it is easier to fit than the original but feels a lot heavier, probably bigger gauge steel.
Cheers
Dave