I still use polystyrene pizza bases.
I have a Waitrose charity counter (one of those green things!) which I draw round. You can get five bulls on a 12" pizza base.
They free stand, can be pinned to a post or whatever with one pin if needs be, are waterproof and dont tear.
They also hold the shape of the bullet hole even if you manage to cloverleaf. no second guessing if that is one shredded hole or two in the same hole. (although that particular thought process doesnt happen often!)