On an HG20 hub, it's a metal dust shield. The cone may have a couple of plastic split-ring seals in a recess, like piston rings in an engine, that sit in the opening of the metal dust shield to form a basic seal, but I can't recall if HG20's have those.
The one on the driveside is different. If you toasted the non-driveside dust shield, the best source for a new one is another HG20 hub. They're easy to deform, so you have to remove them very carefully using a minimum of force. My favorite method is a Cal-Van slidehammer bearing puller and lots of patient ginger tink-tink-rotate, tink-tink-rotate action.