What tools do you have access to, what's your skill level and experience, and what is your budget?
The answers to those questions will determine the best route for you.
One of the best methods for visors will be a Vacuum Formed visor. Since your helmet is 3D printed, the file maker might have included the files for the Buck, or positive mold, for the purpose of Vacuum Forming a visor that will fit perfectly with the helmet, especially if it's scaled and printed the same as the base helmet.
If you don't have the skills or equipment to Vacuum Form, there are folks here who will take commissions and make a visor for you, especially if you can share the files, or can print and send them the visor buck.
You can also go the route of using flexible visor sheets, available on Etsy, or using Motorcycle or Scooter Visors, or even safety face shields and reflective window tinting.