It occurred to me that I needed a color-neutral filter that is homogeneous on a as thin as possible foil. Some hard thinking yielded the idea to use automotive window tint. This material is completely homogeneous, and, a great plus, it is available in a variety of absorption coefficients. I bought a roll of "WINDOW TINTING FILM 50%" on ebay from a seller named "jollefemme" (after testing some darker tints from a local shop which did not do it for me), and it turned out that this worked perfectly. The additional benefit is that this material is also thinner than the safmat foils, i.e. fits more snugly on the LEDs.
To kill the light coming through the back of the PCB I put a sheet of 95% tint on the back. The pics show the difference between front and back filters: