How to fill large window gap for temporary air conditioner?
I think the simplest thing would be to buy a sheet of foam insulation from the home store.  It comes in large sheets so you'll probably have to cut into pieces to get it home, but its easy to work with.

It ticks all your boxes:


Cuts with a razor knife or hand saw
Very lightweight
It literally is insulation
Water proof foam
Well, a determined cat could scratch it up or eat a hole in it, but they would have to really hate the foam. It's pretty durable.


The downside:  One style is pink.  The other style is usually yellow, but has a white printed side and a shiny foil side.  They can both be painted with exterior house paint (just buy a color sample size of paint), but the paint might not be super durable on that surface.

For installation, duct tape will work well, and duct tape comes in a variety of colors.  If you don't want duct tape to leave residue on your window, you can put down a layer of blue masking tape to protect the surface and then duct tape to that.