I will make improvements to the bucket lids. I nee...

Hello VitaMan, You want to use food grade materials. If it is safe for food then it is safe for a product that you grow for consuming like cannabis. I like to use food grade stainless steel for multiple reasons, it blocks the light, it never gets old and it is safe for your product. If you are searching for a 1 time cheap fix then I would recommend aluminium foil that you use in the kitchen. Happy Growing Buddy!

Epoxy will leach out. Will it be toxic to your plant? It will depend on the epoxy you use. I would suggest food grade silicone sealant.

once the chemical reaction is done, yes its safe. The only thing that then could happen is that your grow LEDS destroy it by time if you dont give it a solid paint. It releases the softeners from the resin, just like in cheap plastic.