I dont have any experience using reflective material to keep your tent cool but I really dont think It would be worth it. In my experience the biggest factor is creating a large area of shade with maximum ventilation. Multiple EZUPs together with a white sheet on the east side (sun rises) keeps you cool pretty late into the day. Using white canopies keeps them from absorbing sun light almost as well as mylar would any way. Then you can put your tent under that if you really want a tent but no tent works best. Or a tent with tons of mesh for ventilation.