So Berezin also sells a "pinsharp" device for 50x50mm slide mounts, which has a 47mm focal length or +21 dioptres. It's intended for much smaller images than the Owl and Holmes though, which would explain the difference in focal length.

I don't have any closeup dioptres or I'd just try them. I do have some enlarger lenses (of focal lengths similar to what I shoot with) though so I'll start with those to see how it looks and whether I get enough image circle from them.