A plastic or burlap feedbag stuffed with old plastic grocery bags works good, too, and is pretty quiet compared to a box. Its dead quiet if you stuff it with old clothes...but heavy.
Throw the bag out on the lawn or in the woods and it can be shot from any angle, or hang it between limbs or poles for a simulated 'lung shot' profile. Can even draw the 'leg pocket' on to help pick a spot. Lasts a long time, slide another bag over it when shot out.
Have to stuff the feedbag tight and it takes alot of plastic bags (or plastic wrap), but what better way to recyle those otherwise useless mounds of grocery bags you get each month?