I put two emergency blankets on the deck and fastened them with duct tape. They increase the amount of sun coming in the window, and therefore the heat. Each blanket cost $2. The effect is small, but it was cheap and easy. The house is brighter now too.
You could make shutters from plastic panels and cover them with the shiny blankets. Not sure how you would fasten them to the window though. Every window could be a solar oven.
Instead of reflecting in more sun, you could harvest what you get. Venetian blinds made of aluminum would work. Metal absorbs solar heat. Of course you would get less light. Or get some non-slip rubber material that holds area rugs in place and hang it in the window. It would be like the dashboard of your car - hot in the sun.
Other ideas here: