In my unprofessional opinion the visor isn't really for protection, it's mainly to keep sun out of your eyes so I felt safe epoxying mine back together.
WARNING: This will void your warranty and crash replacement policy.
Step 1: Epoxy
Don't worry too much about the roughness, you can sand it after it dries.
Wait the appropriate amount of time according to your specific epoxy label.
Remove the visor from the helmet.
Then apply epoxy to the inside of the visor.