I had two different models, each for several years. I found them heavy and bulky to carry, and not overly accurate.
For the past 20+ years I have used a gray card with a hole cut to 1/2 the dimension of the film I am using. (4x5 become 2 x 2 1/2, 5x7 becomes 2 1/2 x 3 1/2)
The 4x5 card can also be used for the 8x10. The 7x17 has its own card.
To the corner of the card is attached a string. measuring from the card, there is a knot at 1/2 the focal length of each lens. By placing the appropriate knot against my nose and viewing the subject I can quickly and easily choose the correct lens, and composition.
Such a viewing card was good enough for Ansel, so it is surely good enough for me.
When one wears out after several years, or becomes lost, I just make another. I have bought a lot of film with the money I got for the Linhof finders.