Gene,, The conveyor belt or also known as mining belt is a thick rubber belting material and works good to put behind straw bales to keep the arrows from passing through,,, thats what I consider a "Back Stop" something behind the target.
Mullet, do they come completely off ? Might need some new straw or get a ratchet strap and put around the center of the bales and crank it tight.
wvflintknapper