This is for where the darkslide slides in and out. I have heard of using black yarn/strings or foam for some things but I think that would be best for where something simply closes. The darkslide pushing through and sliding past these things seems like it would crumble them, tear them off or otherwise damage them. I feel like we are thinking of different things, maybe this would clarify.
I know Graflex-style holders had a felt or cloth strip with a thin sheet of spring steel inside of it. That looks like it would be difficult to make for such a small plate holder. Mine are 6.5X9 cm (2.5X3.5 in). The ones already in there are very brushlike.
Sorry if I wasn't clear, I should have mentioned that above.
Brian