The originals have a tendency to be shrunken a bit. When new one could easily fit rods in the top and a cartridge in each of the other two openings. 'New' was a century ago.

My 1922 proofed M1910 TD has the original insert which is distorted a bit at the top hole for the rods to tightly fit if put in just right, other two are too tight for 9.5X57 cartridges. I could bore them out a bit, but have kept her original.

For function, and as your MS will be a 'restored' example, I'd seek out a suitable material of original thickness, cut to shape and bore appropriate holes. May take a few runs at it, but if you succeed there's another valuable MS part you'd be able to reproduce for others.

I don't know what sort of what was used on the originals, just that it shrinks.