Malco’s duct folding tool doesn’t look like much, but it allows you to evenly fold sheet metal for forming various duct parts. It’s simply two steel plates joined together at the center. You insert the sheet metal in the gap between the plates until you see it through the diamond site holes, then twist the tool to form the bend.
Malco sells five versions of the folding tool: 8″ and 12″ long folding tools that make 3/8″ and 1/2″ bends and 12″, 18″, and 24″ tools that make 3/8″ and 1″ bends. The longer the tool the more uniform the fold; the 24″ version can even be used as a mini brake for flashing and siding.
You can pick up one of these folding tools for between $10 and $15.