Mylar and sunlight too much for a plant?

Say I want to make up a grow box and plan to line the back and side with mylar that wont get direct sunlight unlike the front of a plant. Will this be worse for the plant or better? I plan on having non see through greenhouse type plastic on the front. ventilation shouldnt be a problem.

Sounds like a Solar Oven

Often times, growing from sunlight through a window indoors, doesn't supply enough light for the plant. Unless the angles are just right. But, assuming you have enough light, with proper air circulation, it should be just fine with the mylar.