Mylar Film and Panda Film are two great options for sealing light in your indoor grow. Mylar is a highly reflective polyester film, and Panda Film is a polyethylene film that is also reflective.
Mylar and Panda Film can be used to line grow tents, walls, ceilings, floors, and more. Panda film reflects about 90% of light and mylar close to 99%. They both do a great job creating light barriers for plants sensitive to external light.