There are reflective covers, but usually the reflective side is down. You could try it upside down. Or put in solar panels.

"Solar panels??? Are you nutz, Carl? But I want to cool it, not heat it!"

Right. Run the solar panels at night instead of during the day and they act like car radiators, bleeding off heat! Then, when the season gets a little cooler, you can run them during the day and extend your season.

Carl