I'm with Jage on the plastic hullabaloo. However, if interested in a tank, it might be possible to use a RCI aluminun fuel tank (new, of course). I suspect you'll have to do some major flushing of the tank in order to make it safe for drinking (if possible). I bought a 19 gallon rectangular one off E-Hell for under $200 (comes with a GM sending unit too). Measurements were 12x12x30.
http://store.summitracing.com/partdetai ... toview=sku . You'd have to rig up a way to fill it and might need to cut off the fill spout to fit under a gaucho. I remember the early alumn beer/soda cans before they were well-lined. They sometimes had a slight metallic taste. However, those liquids were often stored months in the cans. With an alumn fuel cell, I'm guessing you'd get that metallic taste, but if you filled before your trips and drained afterwards, I doubt it would be much. Don't get me wrong, beer is always better from a bottle.