To begin: cut two pieces of board, each about 8" long. One of these is the cross-piece, which will hold the first-surface mirror, and the other one is the binocular holder.
The mirror crosspiece remains almost as-is, but I cut some wood out of the binocular holder portion:
Notched the cross-piece a small bit for better fit. I didn't have a chisel so I used the "al-Qaeda 9/11 Special" (box cutter) to notch the wood appropriately.
Two mock-up views of how the completed mount should look like. I attached the cross-piece to the main piece with two cheap nails and some Henkel Liquid Nails (TM) that I had lying around from my previous telescope boondoggles. Incidentally, "Ver-Nel" fabric softener is made by the same company that makes Liquid Nails.
I attached the first-surface mirror (a $7 item from Surplus Shed) to the cross-piece with some silicone gasket I had left from some misbegotten car repair attempt. Other similar adhesives would work, too. But not "Vulca-Seal" and its ilk. The Vulca-Seal-alikes are far too tough and almost impossible to remove if you want to reuse your glass somewhere else.
After painting and finishing, I attached a T-nut to the underside of the mount with epoxy putty (outlined in yellow). This shall be where the tripod 1/4" - 20tpi screw will be affixed, as seen in the succeeding image.
And the final product:
In actual use, the binocular mirror mount works (mostly) as advertised. However, I immediately ran into a complaint which users of the more-expensive mounts have noted: the "microscope viewing angle" is only applicable over a very narrow range of altitude angles. Beyond that, you have to adjust the height of the tripod in order to keep the viewing height convenient. And you still have to crane your neck.
It seems that the Trico Machine Sky Window (also here) which incidentally was invented by a Jesuit - is the best over-all solution because you tilt the mirror (much like the mirror underneath a microscope stage). So the viewing angle of the binocular is fixed.
The reason I didn't do the Sky Window style binocular mount is because the first-surface mirror I had on hand (which I bought about three years ago from Surplus Shed) is only about 3" x 6" in size, in fact I believe in my current mount it already vignettes the 10x50 binocular. And to do a "tilting mirror" type of mount, you need an oversized mirror, which I don't have.
But I think I'll try to go for that, I saw some fellow on e-Bay selling first-surface mirrors relatively inexpensively. Or I could call the local scientific houses and see if they have some.