I use this to test material opacity in both flat & vertical planes when printing something that needs to be opaque (like schlem's terrapin pinhole camera).

It is 3mm thick and has two circular indentations in each face (one is 1mm deep, the other 2mm deep).

Shine a bright led type torch against the face, if you get glow anywhere, you aren't going to be light tight at the infill percentage you used. Increase & print again.