However, RobotGarage on etsy is selling 3d-printed S10 lenses for $18, which is a cheap price compared to standard S10 add-ons. I ordered a pair.
Delivery to Europe was quick, and immediately upon receiving the caps I tried them out with my Regulation S10 hooded gas mask.
The caps fit nicely, they are not too snug but will not come off spontaneously no matter how much you shake your head. They are offered in two colors, black and blue: I originally wanted blue but RobotGarage advised me that they might leak some light, and suggest a compromise of blue color with black backing.
The caps in this configuration block light fairly effectively. There is a faint, almost imperceptible streak of light near the rim, which is probably due to the blue plastic letting a bit of light through. This is not a major problem, and you can only see the light if you really concentrate on it.
It’s probable that an all-black version of the mask does not have this issue.
At $18, they are a steal and highly recommended for any discerning S10 owner.