I purchased one of these off eBay for a 3.5cm lens I use with my Nikon S4 (the S4 does not have lines in the camera for 35mm only 50 and 85), I can only report that you pretty much get what you pay for and I guess that better than guessing. https://www.ebay.com/itm/275458461425
Very interesting and potentially useful, but what do you mean by "I can only report that you pretty much get what you pay for and I guess that better than guessing"? Are you trying not to say that it's really not very good?
Compared to the Voightlander 25mm viewfinder it's night and day, it's small, no real guides inside the frame and it is not especially bright and just sharp enough. But its $20 compared to $200, so I guess if the 10% thing is applied, its just good enough for me as I don't use the 35mm lens a lot, if I did, I might spend the $$ for a better viewfinder.
I purchased one of these off eBay for a 3.5cm lens I use with my Nikon S4 (the S4 does not have lines in the camera for 35mm only 50 and 85), I can only report that you pretty much get what you pay for and I guess that better than guessing. https://www.ebay.com/itm/275458461425 View attachment 350533
The quality matches the price: it's like the viewfinder from a disposable 35mm camera from the drugstore.
If you want something better but stay below 100€ or so you can always go off of a P&S camera with a decent viewfinder:
If you have a dremel and a dead P&S camera with reflective framelines (Canon Prima 5 or anything similar), extract the viewfinder assembly and fit it into this housing. I think I saw an stl model for a 3d-printed viewfinder housing for this size of vf somewhere. Can't find it right now though.