My Grow this time around is in an old basement 4'x3.5' shower stall. I lined the walls with reflective insulation, but, of course, the front side was open. I'm not worried about light leak/invasion so I don't need a real door for it, but it did mean a 'dark side' for any plant on that side of the shower.
I took one of my older, big, photography reflectors down and use it on the open side. It's light-weight, easy to roll out of the way, easy to wipe down and keep clean, and, of course, it is highly reflective. Makes a great 'door' for the stall!