Hey all,
Here are the STL files for the two different waist-level optical viewfinders I designed. I'm not the best designer, but they work and are free so hopefully it helps you out. It's a fun project with an exciting result!
18mm lens version: https://www.thingiverse.com/thing:6925336
24mm lens version: https://www.thingiverse.com/thing:6925348
Happy Snappin'
-james