We thought about making some curtains for a while but decided against them for several reasons. First, they do not insulate a lot and second, unless you have some thick felt curtains they do not block light very well (yes we are want to sleep in quite often :).
Therefore bought some regular silver coated bubble wrap from Home Depot, cut it in shape of the windows and wrap the edges in foil tape. The pieces can be folded or rolled together. These covers bring also down the temperature significantly when we park our car in the sun and provide us with more privacy than a regular curtain.
P.S.: The bubble wrap is also perfect to use behind your back when you sit near a campfire at night.