The best deal is for a Devol. I think they are about 30 dollars. Mine has been bulletproof. Try chaparral-racing for best price. Axle bolts through it.
Think of a plate. Bend one end to make an edge. This is the deflector part. Then put a hole in the other end. Slide this down the axle and sandwich between the rear disc and the swingarm. And there you go, you have yourself a disc guard.