This is a cone die. It’s two lengths of 1.5″ solid round bar @ 24″ long. The bars rest on pieces of angle iron that are raised 2″ up from the base. The die allows me to hammer sheet metal down into the center space. The slanted configuration of the bars cause the sheet to roll upwards into a volumetric cone form.
I built this several years ago after attending a sheet metal forming workshop at Haystack.