[Dylan16807]

If you need to deal with heat, you set up one side to be good at reflecting, and the other side to be good at radiating. That doesn't require perfect materials. It doesn't even require particularly great materials. For the job of shading the earth, you would do fine with a design that only reflected half the light. We can do much better than that with existing technology.

https://en.wikipedia.org/wiki/Solar_sail#Materials talks a bit about which designs can handle getting significantly closer to the sun than Earth. None of the designs there would have trouble with heat if they were in orbit around Earth.