Mounting discs are available from Omega Satter. I am
amazed that a machine shop can't make a disc. I've made
many, you need a drill press and a fly-cutter, a lathe will
also do.
  The disc is made of aluminum sheet. The center hole is
cut to the right diameter with a fly-cutter or hole saw (I
prefer the fly cutter) and two holes are drilled for the
mounting screws. Paint the disc with Krylon Ultra-Flat
Black, available in hardware and paint stores. Since you
have a disc as a pattern it should not be difficult. Its so
simple you could make it with hand tools.

Jim

I just purchased a used Omega D2 enlarger and separately a Vivitar VHE
135 mm lens.

Send the disc and the lens diameter and I will cut it out for you.

Bob  AZ

The poor-person's alternative is to spend a few dollars on a round file
(steel cutting, not a wood rasp) and do it yourself. The hole probably does
not have to be perfectly precise. When I was poor I did amazing things
without even a vice, bench, or any machine.

I would double check there isn't something on the lens that is keeping it
from fitting.  Usually, if the hole is the wrong size it is by a significant
amount, 10mm or more.  Is there a sleeve, lock nut, shoulder gizzy in the
way?

That said,

Any machine shop can do this job - they may have scared themselves
into thinking this was some sort of high-precision optics job ...

If you take the disc with you so they can see what needs to be done
they will take the job (unless it is some high-volume production shop --
you are looking for a half-dozen guys in an industrial park or an
old codger in his garage).

I make (made) my own lens discs/adapter bars (bar with a smaller hole)/
lens carriers.  I use copper plated circuit board material.
It cuts by scoring with a utility knife and snapping over a
table edge.  Easy to cut with a hole saw/fly cutter/file.

Jim,

  If you are looking for a cheap source of discs, find your local computer
geek and ask for a free bad hard drive (Std 3.5" for desktops).   The
aluminum disks in there are almost an exact fit for the D2 and they already
have a 25mm center hole.   all you need is some patience and a few small
Torx bits (t5,t6,t7,t8) to get them out of the drive.   I have several on my
D2 with 25mm, 32mm 39mm and 50mm holes that were cut with a hole saw in an
electric hand drill.    There are also Greenlee chassis punches available
that will do the same job.   Good luck

Lynn