Like others here, we went to HD and bought that roll of bubble foil insulation. It just slides under the curtains when needed and really worked while we were in Sacramento heat. Cut it to fit then when not in use, store behind the sofa.
When the rig is stored, we glued the foil onto a piece of cut to fit heavy cardboard, painted the cardboard white and put that in the window. The curtains don't get faded, and it looks pretty good to the neighbor. The only drawback is that the rig is dark inside.
These things work well when cold also.