This is a clip on attachement to my wooden window blinds, it allows me to hangs a sheet to darken the room.
I have a projector and a screen, but the sun shines through the blinds degrading my viewing contrast during the day. This allows a dark sheet to further block the sun without permanantly changing the blinds.
My contrast is much better, while still allowing me to easily open the windows.
Print 6 of them, or the plate file. They press fit onto 2" x 0.1" wooden blinds. I put mine on the top, bottom, and half way point on both side edges. Then I cut a dark sheet 2" larger than the window on all sides, poked holes where the clips were, and attached to clips with mini zip-ties.
As the blinds are raised, the sheet travels up with it and tri-folds, allowing most of the window to be opened for sunlight. An iron can be used to set the fabric, and will improve your tri-fold action.