I use one of these for 35mm ;
https://www.ebay.co.uk/itm/Voigtlan...737053?hash=item5960991f1d:g:zqgAAOSwbCRftvzs
Don't get that one though , it's priced at about double the usual price !
It's just that it's the only one I found on a quick search to give an example .
It's unusual in that you can't see through it . It has a while frame and central dot that illuminated with ambient light .
You use it with both eyes open , so one eye see's the scene , the other eye see's the frame markings , and your brain puts one over the other .
The result is a big bright viewfinder instead of squinting through a tiny one .
It's a shame they didn't do them for 50mm , 85mm and 135mm . Or if they did , I've never seen them !