Aluminet is a reflective mesh fabric (designed to be used on greenhouses) that does a great job of reflecting most sunlight while still allowing some airflow.
We've used Aluminet for the past few years as a shade cloth for our van. It allows us to sleep in despite the intense morning sun, but still get some breeze (especially with a fan).
It's strong. Sharp objects can poke holes in it, but tears don't develop. It does attract dust quickly, but that hasn't been a problem.
Our Aluminet is secured with rope using special clip-on grommets, but others have had success simply bunching up the corners and tying it in place.