I’d use a piece of insulation foam board, cut to fit the window. Lighter than something like plywood, way more insulating, and can be easily squished into place once it is cut to size. Just hole the board up to the window, mark where your cuts need to be made, line one side with shiny foil, tape it along the back, and you’re done. Now you have an insulating reflective panel that can easily be popped in during the daytime heat, then pulled out at night.