Yep, missed your original post on that. I see it now. My bad!The slider underside isn't flat, and neither is the housing. There are some longitudinal raised lines along the slider, and also in the housing. They are not very protruding, but putting something there would probably make the slider not slide as smoothly.
Edit: I actually had tried that, and it didn't work for me because of the friction. Maybe it needed to be an even thinner piece?
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