All good ideas and info. I ended up finding one today with the foot adapter so I should be good to go. My responses to your points are below for the sake of conversations. Thanks for your replies.
Another approach would be exchanging the orginal, big foot by a standard foot (taken from a rangefinder, flash, etc.)
I considered it but this would necessitate making a plate to attach the standard foot to the barrel of the finder and match the hardware pattern. Probably more effort than just making a slide in adapter which I also considered. Prefer not to have to fabricate in this case (enough of those kind of projects already — hoping for a more plug and play solution here).
Can't help you with the Horseman, but there is a Linhof finder that does mount on a standard cold shoe. I use one (not connected to anything) for scouting locations, before pulling the tripod out.
Yea, I’m familiar with the Linhof finders. That’s what I was initially going to go with but the shortest focal length any of the Linhof zoom finders go to is 75mm on the newer ones. My shortest lens for this project is 65mm which the Horseman matches which is why I’m going with that one.
They made a 'handle' for it also, to use as a 'directors' viewfinder.
Yep, I almost grabbed one of these with the plan to scavenge the mount off the handle. No longer necessary though since I found one.