I hunted feverishly for a solution. I talked to my local machine shop & an electroplater who does custom car & motorcycle parts. I settled on have about 20 aluminum blanks made at the machine shop ($100), and then polishing them in my shop. Aluminum polishes very easy with a drill press & a Ryobi polishing kit from Home Depot. It is also easy to tool for a one-off jack plate. Just buy a small chunk of 1/16 plate, trace a standard strat plate, and cut it with a coping saw with a metal blade. Be careful with what you use to deburr it. A wire wheel will scratch the heck out of aluminum.
Electoplaters get $100 minumum typically. I also looked at home electroplating kits, but didn't really want to get into that project. I have a design scheme for a fancier model, but need time apart from other projects to complete it.
Hope this helps. It would be great to have some of these things commercially available again. Eventually I'll put in a milling machine.
"Hi Ho...the Carrion Crow...FoldeRoldeRiddle"