Next time out yarding I can grab a base and rod for you although it will probably be pitted like your original rod appears to be. You can swap your original mirror onto it by removing an allen bolt.
This was in a box of parts with a 72 Dodge I recently acquired, I'm guessing it's the front speaker mount? Sounds like it's kinda along the same lines as the custom bracket that you're talking about.
Have a busted cruise control switch and was able to snag this extra stalk from a yard. Hoping someone can give advice on how to properly dissasemble this brittle plastic so I can make a one good switch out of both