Window coverings, such as blinds or draperies, are more effective at reducing summer heat gain than that of winter heat loss. However, insulated window coverings help inefficient windows keep the heat on the inside.
The most efficient insulating blinds contain thermal lining or material which creates tiny air pockets to provide the insulation and seal tightly to the window frame.
You can cut a piece of rigid insulation to fit your window and cover it with a piece of plywood, then paint the plywood to match your walls and add knobs for an easy fit.