It turns out that I did not measure the diameter of the base of the mirror cell quite carefully enough. It is actually 20.25"; the inner diameter of the Sonotube is just barely under 20" (probably because of the resin). This is a bit big for me turn myself, so I have to find a machine shop Monday that can turn this down without charging a ridiculous amount of money. It doesn't need to be terribly precise: 19.875" += .05" sufficient.
I did take some more weight off the finderscope base; it is now down from its original 5.2 ozs. to 1.2 ozs. I don't know why milling produces such bad results; oil helps, but not enough. The flycutter does a nice job, but an end mill just makes it catch and stop pretty often. I sometimes wonder if this vertical mill, which I bought used, has some problem.
UPDATE: Good news: a machine shop in Eagle quoted me $20 to turn it down to size. That's a bargain.
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