I bought a 35mm finder to use on my R-D1 with a 25mm lens, but I never actually bothered. The whole built-in finder is good enough, and let's me focus and frame at the same time. I use a Canon 25/3.5, so RF focusing is possible. For the non-RF-coupled CV Snapshot Skopar, an external finder is probably much nicer. Get a 28/25 CV minifinder and you can use it for both 21mm and 25mm lenses.
Ari