Bright line 90mm finder. These in any focal length are better than any M camera frame line. I have a full set of original Leica + a CV 75. I will never part with them.
Or an imarect which is continuously variable, so you can make it perfect. Problem is the optics are uncoated and the image looks dull. But it very accurate with masking blades. Say you like a 40 mm lens, you can mark off where to set it for to match that lens.