After trying scores of boards with different form factors, layouts, and switch types, my current favorites are as follows:
IBM XT, RF 87u 55g, HHKB Pro 2.
Based on my preferences and research, I think that I would like to do the following to get close to my dream board:
HHKB Type S with 55g domes from a RF and a black case.
This dream board could be cobbled together with parts from other commercially available boards.
Another one I would like to find is:
IBM Model F 62-key "Kishsaver".
The Kishsaver exists, but it is rare. I am still looking for one.
A dream board that could not be made easily:
HHKB Type S with black case, 55g Topre switches, and a steel plate. Perhaps with a Bluetooth option.