Aluminum. I believe MacoJoe has pics in the gallery of one he made of steel but then bought an aluminum one.
http://www.wellcraftv20.com/gallery/album91/Picture_027
Don't know that i'd undertake learning to weld aluminum as I hear it's an art. Better off finding used.
New they seem to run 1300-2000. Managed to get this used for $600.
Powder coat is excellent. Can't wait to see it mounted.