here in florida, (hurricane territory), we have springy metal clips that hold plywood in a window. i use them for my darkroom. i have one window covered by a piece of exterior grade plywood and 4 clips hold it in place on the outside. voila, no light and the bright florida sun is shining on that window ALL day! no tape, foam caulking - light proof. they are easily removable but wouldn't recommend if it's something that you put up and remove frequently.
i had my stepson paint a mural on it, and looks great - works great - all happy.