@agus35monk I am using 55mm ping pong (oversized) ball, which is decent for an 8mm target. No coating was used, I did select that the visible portion does not have the hemisphere seam or the injection mark.
@justin thank you for sharing these references. The last reference uses this interesting aluminum cake molds, which are available in various sizes. The part looks like a promising half sphere, perfect for light sanding and coating. As mentioned by @cpixip, the relation between the diameter and the port size, and the reflecting surface are critical.
As mentioned prior, imageJ is a great tool to measure the resulting flatness of the illuminant+reflector+lens+sensor+library and provides a great visual representation to confirm the system is flat.