Hand warmers, placed over the toe box.
Light weight, pack easily. When you're done glassing, put them in your pockets to keep your hands warm
Tape them, or cut an old sock so it holds the warmer on the toe box.
I hunted in an unheated blind for 60hrs in Saskatchewan. I taped the hand warmers on the toe box of my pack boots, then put on artic shield over boot. Didn't got cold feet...or a buck, btw....
Anyway, only way I've found to keep my toes warm sitting. You can actually feel the heat coming thru the toe box.