One possibility is using thin (approx 5-6mm) plywood. 3 sheets, each long enough to fit snugly in the window frame top to bottom. Each just slightly less than 1/2 the width of the window. The third sheet covers up the gap between the other 2.

The reason for 3, instead of one full size sheet, is that it's easier to put up and take down. Plus easier to store. You might want to paint the sheets flat black to minimize any reflections between the top and bottom sheets.

For this to work you need a reasonably snug fit at sides, top, and bottom. Fabric, wrapped around the outside edges on the plywood, can help.