I would be wary of anything not designed for a lens. Those look like they are intended to be used with lighting (high temperature) so they probably aren't optical quality. I don't know how it works at the rear of a lens, but I know if you put a cheap piece of plastic (even if it looks perfectly clear) over the front of a lens, it will completely ruin the image quality because the plastic has distortion and waviness that you can't see but that gets magnified by the lens.