I've got a couple of cases where by necessity I am shooting through glass and I cannot use any of the tricks mentioned as answers to questions like this and this. In both cases, I cannot change the position of the camera (in one application it's mobile and the position changes such that sometimes there is a problem and sometimes not) and there does not exist any kind of (e.g. polarizing) filter that would fit on the cameras in question.
The best solution I have found is to put some kind of black material on top of the reflecting surfaces as that is the one variable I can control. But the problem is that I have not found any kind of black material that is actually completely non-reflecting. I have tried black paper and cardboards, but have found that I could "see" both of them reflected in direct sunlight. I recently went to a fabric store and found the darkest black material they had, of cotton (as stuff like polyester tends to have a reflective "sheen" on it), and of a very fine weave (so the weave isn't large enough to show contrast). But it also had the same problem - at the very edge of where the fabric reflected I could see that the black surface produced a noticeable increase in the white level of the glass. If the fabric isn't completely smooth I can also see noticeable contrast differences in this reflection.
I realized that this makes some degree of sense - even with for example a 99% light absorption rate, 1% of bright sunlight is still quite a bit of light - as much as 10 watts per square meter. Unfortunately fabrics don't usually advertise their actual albedo, and I don't want to spend a lot of money ordering various fabrics only to find they don't work! It's also possible that a material that has an internal structure that isn't parallel to the surface might work but I don't want to spend a lot of time building and fine tuning (for example) a piece of cardboard that has pieces angled up from it.
I was told that certain photographic fabrics or materials might work, but unfortunately they had never heard of this at the fabric store, despite the fact that that was practically all the sold in the entire large cavernous store. It's also not very suitable to try to determine reflectivity under the comparatively (to the sun) dim lights of a store.
In my searching for a suitable material I have read about various things like "flocking paper", fleece, "Duveytyne", velour, and a few other materials, but these were in the context of very dark materials for photography not specifically for reflection elimination, so it's not clear to me if any of them would work. What's my best option for a black material that is as totally non-reflective as possible?