Warning - I am a sewer and creative
38 years ago we bought an old house with plans to renovate 'in the future.'
We had very drafty windows, I had lots of time, so we (mostly me) bought some sticky back velcro and put one side around the window.
Then I sewed something with the mate of the velcro on the back, so that air coming through was really diminished. I used what was sold as 'insulated batting'
I made it look like a quilt, went with the decor.
On one, I even made it look like a window with our pets looking out.
In the spring we took it down and the curtains/valance hid the velcro stuck to the wall. We used them for about 5 years, new windows were part of the 'adding on' project.
Too bad the kids aren't little now, just found the Santa peeking in.