I'd suggest black plastic, plywood, rigid insulation, aluminum foil, etc. Is there something tricky about the window that would lead to a less obvious solution?
What I do is put up some old blinds and make sure to close them. Then I place the black plastic over that. That way from outside people think it is just a regular room. Boarded and plastic up windows are sure signs of a grow in progress. Just to be safe. It's obvious, but I had to say it.