second window for toilet on rented apartment
Window insulation film.

https://www.youtube.com/watch?v=gwkmgbzGG-g
This window kit is clear plastic sheeting.  You attach it with double sticky tape along all edges.  When I did this the last step was to blow hot air on it with a hair dryer which got rid of wrinkles.  3M makes a kit.  The plastic must be something like shrink wrap but I dont know if that is essential.
In my old drafty 1930s house I made adhoc extra windows with sheets of plexiglass that I cut to fit the windows.  I used foam pipe insulation along the edges and pushed them into place.  They worked great but the windows were big and the plexiglas was pretty expensive.  I toted those pieces around for some years after moving hoping to find other uses for them.
If I were going to do that stuff again with maximum cheapness I would skip the kit ($$$) get a piece of clear plastic sheet (¢¢¢) and put it up with double sticky tape.  Then in spring carefully fold up the plastic for next year.