Brilliant cool-weather problem-solving, Jessica!
My toddler wakes every night crying because she kicked off the blanket and is cold. I took another crib sheet and cut the elastic away except for the two corners on the ends. I tuck those corners under the mattress and let the rest of the sheet cover her. She can't kick it off as easily and it is lighter-weight than her blanket, so she stays warm (but not too warm).