Custom tonearm boards - who makes them to spec?

Normally I would suggest the TT manufacturer but I understand TTWeights was sold off to another company(?) They might be the ones to contact.

I would also suggest avoiding Delrin if the arm board is cantilevered. It’s nowhere near strong enough (even the thickest piece can warp) so my advice would be to choose metal alloy.
The alloy can also be threaded which is a big advantage. ;)
Hope this helps...

FWIW: I built a prototype out-board motor enclosure out of birch ply for my upcoming knock-off of a VPI DIY tt. After 2 years of testing I had my local machine shop make the top plate out of 6061 aluminum with pretty intricate details. They even had to make their own custom bit in order to make the finished radius on the top of the plate edge. BTW, they let me supply my own material which was a savings.

When I went to pick it up, I almost went to tears because, in my mind, it was absolutely beautiful. It was perfect!

I found this place online.

https://fosworld.wixsite.com/magna-audio/armboards-pods-plinths

Haven't used them.

Wow, In my day audiophiles were very creative guys. We made our own arm boards. Childs play. go4vinyl if you send me your arm board I will be happy to make you one out of anything you want as long as you pay for the materials, no labor included. It won't take me that long, maybe 20 minutes. 

Mine, but in Germanyhttps://www.graviertechnik-hupfer.de/vinyl.html