In this case, what came to me was a cap made to fit the furnace duct I'd used as a tube, and the next size smaller cap, which neatly nestled inside the larger. A few thumbscrews, slip on nuts (the sort used to secure auto body parts), a length of aluminum square tube (and a hacksaw, since I didn't own a decent one) -- well, okay, the total outlay was a good bit over $10 -- in fact, over $20 -- but I've got most of the aluminum tube still around, and I don't count the hacksaw into it, since it's a durable tool that should last through many telescope (and other) projects.
About an hour with a nibbler making the hole in the outer ring convinced me that was enough, so I used snips to make the inner hole -- it's a lot less smooth on the edge, but it also took about 45 minutes less to cut.
You can just see the blind rivet that secures the angle cut section of square tube to the inner ring, as well as the three holes with silicone squeezing through that add strength to the bond that holds the mirror to the cell. This design has very good ventilation -- there's an inch of clearance between the glass and the cell, and about a half inch all the way around the glass -- but the baffling is only fair. In daylight, I can readily see light that's come around the mirror and scattered from the blackened interior of the tube and cell. Still, that isn't a big problem at night, which is, after all, the most common time to use an astronomical telescope.
Oh, and one other advantage -- this cell is quite light; without the mirror, it was just under a pound.
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If you have comments or suggestions, email me at silent1@ix.netcom.com