I use several different methods, depending on the size of the finder.
For larger finders (wide angle like yours or the ones with parallax dials for longer lenses) I have plastic slide storage boxes, about 2x2x3 inches. I line them and make a divider with thin, ribbed closed cell foam from the hardware store. The foam is designed to put between a foundation and the bottom of a stud wall in wooden frame construction. It's soft and resilient. I can fit two of the larger bodied C/V finders in each box with double padding between them.
For smaller finders, I use either individual or compartmentalized plastic prescription pill sorting/carrying boxes. This works for my 28/35 combo finder, and my 50 and 75 finders. Those finders don't have protruding curved glass surfaces, so they don't really need padding in the plastic boxes.
Jewelry pouches are fine, but they can be a bit slow if you're changing finders very often, and I like the relief from outside pressure that a plastic box provides inside the camera bag. I changed to the plastic boxes when the drawstring operation of the jewelry pouches supplied with the C/V finders became too slow and cumbersome for me to be ready for upcoming shots.
I'd post photos, but I'm out of available attachment space and can't figure out what more to eliminate from prior postings.
Alternately, you could get a 21mm finder for your Biogon that also holds film. It's made by C/V, with model numbers R4M or R4A. But it costs almost twice as much as the ZI finder.
Lee
Internet disclaimer: I do know the reputation of the 21mm ZI finder and the good reasons for buying one. I need a bit of levity on occasion, and am sometimes forced to supply my own.
