There is a cheat for this. Measure your windows. Buy a roll of heavy-duty Reynolds Wrap and a couple of rolls of clear Duck Tape (packaging tape). You will also need some 30-lb. test fishing line and a couple of small screw eyes/cup hooks.
Cut foil long enough to DOUBLE and still cover your windows' inside surface, plus about 1/2''. Fold it double, and make a finger-crease in the middle. Unfold carefully. Starting so that finger-crease is in the center of the tape, carefully cover the entire surface of the foil with the clear tape (you're looking for a laminated effect). Use two thicknesses of tape across the centerline on the inside to stop the fishing line from cutting through the foil.
Lay your fishing line along the crease, leaving about 6'' sticking out at each end (if you're into foofarraw you can use ribbon or twill tape or something fancy instead of fishing line here, but fishing line is the strongest cheapest solution) and fold the foil closed. Now laminate the top side, flip it over and do the bottom.
Tie a slip knot with a loop in each end of your string and mount your hooks inside the tear wall (HINT: if you stick a hook in each front/rear bulkhead, you get built-in storage runners to hang one of those handy mesh 'hammocks' across for keys, wallets, etc.). Slip the loops over/into the hooks, and adjust the tension to make the curtains as tight as you like against the windows.
They're a little stiff but they insulate and will last a long time. You can also use these as "liners" and put "pretty curtains" over them.