How to block a window with plywood for big wall to project a movie?
I have had great success using a 4x8 foot sheet of white foam board. 

It is extremely light weight. I hung it from ceiling on strings attached to three binder clips or bull dog clips like show below.

When not in use find a place you can store it flat. If it gets broken it’s cheap to replace. If it falls on a child no one gets hurt. The white foam is a good surface for projecting onto.
The one pictured below had foil on the other side but the plastic with lettering peels right off leaving a bright white surface .