This definatly can be done..Any comp with alum fabbing skills i'm sure can build for you very easily if you were to provide dimissions etc.. The cheapest way would be building it yourself or if you or you have any friends with alum welding experiance that would be the way to go if cost would stop you from having a custom shop do the work.. If you were to live closer to me I would be happy to help I actually weld alum for a liveing as I'm certified to weld aluminum up to an inch thick.. This would be a pricey project with alum though finding a shop to do a custom one off tank to your specs. To my knowlege there is nothing on the market that is built for our cars like you are wanting done. Is there a reason your thinking Aluminum? weight,looks? Since it will be hidden under the car you may want to consider haveing a steel tank built v's alum would definatly be much cheaper as far as material goes and your options would be more open to other people and shops that can build it for you..