Cut Reflectix (bubble wrap with aluminum on both sides) to fit into the tent end window areas. Reflectix can be purchased at any well stocked hardware store, home improvement store, or of course Amazon. Then get some space blankets and spring clips to hold it on the tent end walls meet the roof. You can google a product called PUGS (pop up gizmos) too.
Sorry but the Reflectix doesn't show too well in the pic. Here's another pic from somebody else that shows the Reflectix better.