I really have no answers, but had recently gotten into stereo photography. I bought a Realist 45 on ebay, and it looks as if it only had a few rolls of film through it. Still waiting on a real Realist in my price range (hope to get a Custom).
You've probably searched the web and found sites like this:
So I won't post a bunch of links unless you want some.
Anyway, I've seen Holga viewers and masks on ebay, but they'd likely be too small. I'm sure they are not great, but might make a good starting point when building your own (I often modify new toys to suit my purposes).
Considering you will be using 6x7 transparencies, perhaps you could adapt an Holmes style viewer, or the Owl Viewer designed by Brian May. You images won't be that much different in size than a "standard" stereo card, so I'd imagine the optics and distances would be a good starting point. (I'm guessing each image will be 6x7, not the pair combined.)
Are you going to make reels like a giant Viewmaster?