When we were kids my dad built us a little treehouse and this is what he used for Solar Lightbulbs- it works a treat.
Fill a clear water or pop bottle with a mixture of water and regular bleach. Put a cover on it. Install it in the ceiling and make it watertight around the edges by caulking it with clear silicone caulking.
This is great for storage sheds, treehouses, playhouses, etc.
If you’re the scientific type and want measurements- use 10 ml of Chlorine Bleach and then top off the bottle with Filtered Water (you don’t want treatment chemicals in it!)
The best part is that this “Lightbulb” never heats up so the kids can’t get burned on it.