I'm thinking of putting a heater in the stall, because it isn't a fully blocked in weather proof area.
If you don't mind I'd like to suggest that you add more wood to her stall to block the wind. Heaters are dangerous.
Try Lowe's--they sell extra wood on Mondays and Tuesdays, that they sell for $1 or less--I've gotten a LOT of scrap wood for projects including some really nice 3/4 plywood pieces and one 6 ft. 2 x 8 board that cost me 25 cents. (They keep the wood for 2 days then throw them away, believe it or not!!)