Reserve fuel tank?

At some point in the not too far distant future I would like to put a 5 gallon reserve tank on the flat spot in the trunk right behind the rear seat and put an electric valve (normally closed) between that and the existing tank to dump from the reserve tank to the main.
I would like it to be about 4-5 inches high, 36 or so wide and about 18 inches deep front to back and preferably aluminum.
At my hiway mpg this would give me an approximate 450 mile range between fill ups on long trips.
Anyone know of a good source for something like this?

Tank you.:D