Roll a few beads on what's to be the top of the oilbag.
Top support to hold the aluminum to the hammerform without marking it.
Line up and make an mdf and alum sandwich.
Start hammering. Go slow and even as you make your way around, taking extra care at the corners. Use a plastic faced mallet (or some other non-marring hammer) to keep from marring the surface of the alum too much.