A commenter on this Reddit thread says:
"You can get space blankets cheap on Amazon. Cheap like maybe a buck and change, and each one will cover two windows.
"Mist the window lightly with plain water on the inside of the glass. Cut a piece of space blanket Mylar maybe a wee bit big and press it against the glass. Work out the wrinkles to your satisfaction and optionally run a box cutter around the edges for a neat fit.
"When Winter comes and you need the solar gain, just peel off the space blanket and fold them for later use.
". . . I can't remember where the heck I got the Bubble Wrap trick, but it's totally awesome, and it is actually the genesis of this tip. Kept me toasty warm last Winter."