A similar idea is to put a piece of unexposed-but-developed black-and-white film over the viewfinder window (not the rangefinder window). The purpose is to dim the VF image some so it won't overpower the faint RF image. Color film would work too if you don't mind an orange viewfinder. You can also use window film, which comes in large rolls for dimming windows of buildings. That works well.
Mark Overton
Better than a gel on the RF window, because it improves the balance of intensity between the two images. That worked for me to improve the RF experience with a 35RC. Keep in mind that a film substrate with anti-halo/anti-piping treatment might not only
absorb, but also
diffuse some light, which does not matter for normal use, because it is
behind the emulsion. I had not thought of the window film. Other possibility: Rosco E209 gel (0.3ND, 1 stop) or similar from Lee, can be found in moderate sized sheets.
Also, things might improve simply from cleaning the VF optics, but clean the dichroic mirror at your own risk.
I have an Aries RF with very weak rf spot, and will try replacing the splitter with something like this (just an example among many), properly cut to size.