When the family was younger, smallest used to use my alpine bivi waist high sleeping bag, called a pied de elephant . Down filled so quite light and compressable , 48 inches long , 24 wide at top, 18 wide at base. It went to Nepal after the earthquake but I know the dims as I made a lighter synthetic replacement.
Another option is to butcher a synthetic sleeping bag, cut it to size and restitch without a zip, about ten minutes with a sewing machine and scissors.