So, for the longest time I was trying to find a solution for this. One day I ran into something that looks like swimming goggles, and it was completely opaque in black color. And since it is in goggle sized format, it would fit over even one of the largest FFM, the F20.
So, if you need a sleep mask and you use one of those masks that the sleep mask will not work with, I recommend this product:
Wahah 3D Hydrating Moisture Sleep Mask
https://www.amazon.com/gp/product/B08114...UTF8&psc=1
Edit: I just realized these are overpriced sun tan goggles. I would go for sun tan goggles as cheap alternatives.
There is no opening that the light can be let in. The middle portion of each goggle side isn't an opening, it's just a fiber patch on the inside of the goggles. It's used for adding moisture.