I've had good luck using that "double bubble" foil insulation with black felt contact adhesive'd to one side. Made them a 1/2" oversize so they stay on just pressing them up to the glass & running a finger along the edges to wedge 'em in. From the outside it just looks like an extra dark tint & the reflective inside increases light inside the cab at night, not to mention all the improved insulating it adds vs glass alone. They're so thin you can roll them up & stow them outta the way wherever you like. I have one for each window, here's one side with them in place:
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