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You can make your own out of fiberglass mat and resin if your handy, or simply go to a HVAC contractor and have them make metal ones out of galvanized sheet metal. When I had my metal shop we made them all the time for folks in our area.
Most people simply sprayed undercoating to make them resistant to any rusting although the galvanized metal resists rust for many years.
No I no longer have a metal fab shop, although looking for some equipment to do some smaller
You could also coat the inside with fiberglass resin. There are lots of web sites that sell this type of product and its just a matter of mixing and brushing it on. Let it dry and your done.