I have not used heavy duty aluminum foil, but I have used those cheap foil cookie sheets that come two for a dollar at the grocery store. I cut them into 5x7 (or so) pieces, then flattened them out with the edge of a piece of mat board (to get rid of the embossed words and grooves). I taped these with masking tape to pieces of cardboard, and the students transfered their designs and repousseed by tapping thick bolts against the metal. (We used sections of large dowel rods for "hammers"). Hope this helps!
-Lydia de Toledo