I do this with “reflective bubble insulation” brand: Reflectix with a spray adhesive.
Unfortunately it looks like a reflective surface work best with an air gap so directly against the window might not be doing much. But certainly the insulation will.
I think superfoil is the generic name (or possibly another brand).
I used suction cups, with thumb screw threads to attach them. Just poke a hole and push it through on each corner. It also holds it slightly clear of the glass. Suction cups also mean you can apply it either inside the glass, or outside (better, but harder).
I do this with “reflective bubble insulation” brand: Reflectix with a spray adhesive.
Unfortunately it looks like a reflective surface work best with an air gap so directly against the window might not be doing much. But certainly the insulation will.
I think superfoil is the generic name (or possibly another brand).
I used suction cups, with thumb screw threads to attach them. Just poke a hole and push it through on each corner. It also holds it slightly clear of the glass. Suction cups also mean you can apply it either inside the glass, or outside (better, but harder).
That’s a great (and cheap) solution!