Just go to your local sheetmetal shop and ask for a scrap piece of .100" aluminum big enough to match the gasket. Drill it, shape it and put it on with a gasket and sealer. You can use thicker material, but I wouldn't go too much thinner. Doesn't really matter what alloy. Tell them what you want it for and they will probably agree that anything above soft or annealed would work. And, they will probably just give it to you. Take the gasket with you. You never know, maybe they will even shape it for you. Just be humble.
RamchargerCentral.Com is a Dodge truck community built by a vast number of experienced owners. Whatever you're thinking about doing from routine maintenance to full-on restoration, odds are somebody around here already has. Originally focused on the Ram Charger / Trail Duster, our members own, drive and wrench on Mopars from the 40s to the current day. It's not what you buy, it's what you build.