How to add warmth to cold window spot

My girl loves sitting by windows all the time. The problem is, my windows are poorly insulated and grt cold with the blinds open.

I have a spare room that I've kept the blinds open (since I don't use that room) but the problem is it's been getting cold in the room and up against the window.

I have a self heating pad under a blanket in my kitty's spot, but the question is, is there anything else I can do to keep her warm? I do have a furnace that kicks on every so often but that room seems to stay on the cooler side. I don't know if she'll go under blankets or anything as she hasn't displayed this behavior before