The biggest culprit is the blue thingy part of the slider.

Matte paint makes things worse. Matte means it will disperse light, not necessarily absorb it. And if parts of the dispersed light go towards the lens, we're back at square one (and that is exactly what is happening with the blue slider).

So, the best solution (paradoxically) is to make it as reflective as possible but AT AN ANGLE so the light is never reflected BACK. If the blue slider would have been a mirror (or even metallic), there would be no haze (as the light would bounce sideways, not back onto the sensor).