Problem finished - but not solved. I tried /adapted
- an Olympus Varimagni right angle viewfinder: no sharp image attainable; image laterally inverted, narrow field of view
- half of a compact binocular (objective convergent - eyepiece divergent): to bulky, narrow field of view
- Yashica magnifier finder eyepiece: no sharp image attainable, narrow field of view
The main problem is to focus the optics to the focusing screen.
I finally bought a "1.08...1.6x zoom magnifier viewfinder eyepiece": sharp, 'full' field of view; about 18 US$.