Take the shape you have in your head (of the piece you want to make out of metal) and make a template on some thin cardboard. Use the template to transfer your shape to some 3/4" mdf and cut it out. In this case I cut multiple mdf profiles out because I want a top hammerform and a wide edge profile to help shape the sides of the oilbag.
Make your first mdf profile nice and smooth on the edges with a sandpaper/block, palm sander, belt sander, whatever you have to do the job. This is so you can easily clean up the rest of your rough cut mdf profiles using a router and a flush cut bit riding on your first clean mdf piece, making them all exactly the same (if you want to be picky). You can hand duplicate these with simple tools, the router just speeds the process up.
Change the router bit to the roundover profile you want and rout the top edge of your top profile. You can hand rasp/sand to round the edge over if you don't have a router.
Screw all you pieces together and you have your finished form, ready to hammer on.