Fabric stores sell material that is light proof and used for window blackout curtains in hotels. The stuff I got was white but is really light proof (maybe 99% in bright sunlight). I cut my material in long lengths corresponding to 2x the long dimension of my filmholders, and about 1" oversize on the width. I took the material to my local dry-cleaners where they have a lady that does alterations. I explained how I wanted the material sewed. She charged me $20 to do 4 bags as I described. I used stick-on velcro on the material that makes up the flap. Works very well in the noonday sun at altitude.
I agree about using the sharpies to blacken the wood.