I've got 6 large windows in a hallway that I'm sure aren't so modern
(some of them have leaked). Was considering a few layers of clear
plastic, spaced evenly (1/4" apart) and stretched over a wooden frame,
which I will screw onto the existing frame in winters.

I know first hand that plastering newspaper over old windows helped
magnificently when I was painting once in my old house, but I don't
want to do that this time for the whole winter.

Seeing clearly through them is not important.

Thanks for any ideas,

Dean