I second the recommendation of buying a bayonet adapter and the range of filter choices will open up for you. I'm personally not a fan of covering up the amazing Zeiss glass with another piece of glass. But when I do, I really love the ultra thin B+W and Heliopan filters on my 503CW.

I tend to shoot wide open from a lower vantage point and I need to use the magnifier to fine-focus. I do shoot handheld on occasion, but I personally rather a tripod to allow myself to pop up the magnifier, get my head down to the viewer, focus, get my head back up, and shoot. (More difficult for me to do if I'm hand holding and hoping that I don't move the camera). This may not be an issue for you, but it's food for thought. Enjoy!