Custom Focuser Base for 18" Newtonian Reflector

It was time to replace the focuser on my 70's era scope. I bought one from 2047 Studio because it appears well made and fits my budget. The focuser has a separate base designed to retrofit GSO scopes. I thought I'd be able to buy a mounting base for my scope in the commercial market, but there's nothing out there I can find. I need something like a 4" square by 3/4" thick piece of aluminum billet, with an 80mm central hole, radius cut on the bottom to fit a 21" OD tube. Does anyone know where I might find such a piece or know someone who could mill one for me?  Thanks!