I'm trying to come up with a way that could fix light bleeding on caps. I believe it would be best if there was a way to make a thin black shell on the inside of the cap blocking all light from shining through. I wanted to see if anybody had any other suggestions, designs, or current ways to fix this issue. I'm currently to be trying out a few ideas and report back if there is any sort of success.
I cut out a piece of black paper just to see how effective it could be, it worked surprisingly well. It is obviously not a permanent solution. I'm hoping to create a thin mold for the inside of the cap that would cover the entire inside of the cap while having a cutout for the stem.

Any reason why I would not prefer to use any sort of tape, paper, etc. is that it would be ruined if I would ever want to wash the caps.