Yup, medium-sized feed bags. I stuff mine tight with recycled plastic grocery bags, old plastic whatnot, old clothes/rags, etc, and tie opening off tight with wire or twine. Once shot out simply slide another bag over the old one.
Can be hung verticle or horizontal. On horizontal bags I like to Magic Marker on a deer's 'shoulder crease' and hindquarter, with no aiming spot behind the crease (except for arrow holes). Have even been known to paint the bags for effect(leftover gray or brown housepaint or spraypaint works), except for white 'bellyhair'. No bullseye makes it necessary to pick a spot, and when hung at the right height the overall shape/size is not unlike that of a deer's torso. Shoot it from one side for awhile, then turn it around and shoot the 'puffed out' portion back into shape. A cheap and effective 'deer target' if 3-D targets aren't in the budget.