I've even seen homemade blocking plates made out of Hardware Cloth, which is a wire mesh. They also make it in plastic. I wouldn't expect either to hold up for very long, but it's pretty tough stuff...should be easy enough to replace once you figure out have to mount it as well. You could use something like pieces of aluminum from a scrap screen door/window for a frame or to just bolt across to hold it in place. You could even nail it onto pieces of wood with roofing nails and bolt that into your discharge opening.
BTW, hardware cloth works great if you ever want to make a sieve for sifting top soil or compost or to look for buried treasure.
Last edited by Darryl G; 12-16-2012 at 02:17 PM.