Like the idea of buried steel post in concrete, then whatever "wrap" or cosmetic covering goes over that up to your mounting plate. Covering could be cedar wood, pressure-treated wood, "poly" wood, etc or whatever looks appropriate for you that matches the mailbox.
For in town, low mph setup think you have lots of leeway. Being in nice part of town helps even more as sounds like your risk of vandalism is low.
You might even consider stamped steel that has designs like ceiling coverings. If it had small enough pattern, you could wrap base in stamped steel with artistic designs that complement the mail box.
Good luck in dreaming up what would work with that cool mailbox.
If it were ME . . . . I'd consider going "whole hog" and running low voltage lighting out there and put in nice brick setup that matched the house. Having a lighted address and that cool mailbox would look VERY nice in upscale neighborhood !!
