I tried the different finders at Robert White, (my corner shop...). Whilst the Zeiss gives a great impression optically, it is big, bulky and has sharp edges which I feel would be prone to snagging.
There are two CV finders, the cheaper circular one and a lesser known rectangular one which is more expensive, but has a wonderful unobtrusive profile that can happily stay attached to the camera even if the 21mm lens isn't mounted. The optics are fine and the biggest danger I reckon is that it is so small, that it could fall unnoticed from the bag!
I think that this is the one http://www.robertwhite.co.uk/product...ini-Viewfinder
Just realised that this is 28mm, not 21mm. This was right for me as it was going on a Leica camera which has a 1.3 crop factor.