If you're going to get a waist-level finder for your Barnack, you may as well get one you can actually focus with:

Megoflex by
Vince Lupo, on Flickr
This is the Hugo Meyer Megoflex that I used to have. It worked great -- not only could it function as a 'waist-level' finder, but it had a magnifier you could deploy for critical focusing. You could also fold the mirror down and use an eye-level finder on the back to focus.
If you don't want to spend that kind of money on a waist-level finder, a DeMornay Budd reflex viewer basically does the same thing. Oftentimes, however, these types of finders were made for specific lenses (like the 5cm f/3.5 Elmar or the 5cm f/2 Summar to be precise), and in some cases (like the Megoflex above) for specific cameras, so double-check before you lay down money on one.