I just pick up 1/2" steel plates cut in various shapes at the local scrap yard. Typically sold by the pound, you can get stuff for very cheap. I got 12" and 16" circles of 1/2" plate for like 5 dollars. Spray paint em white, drill a few holes in them for mounting, and you are good to go for a long time. When you have blown them to all hell, sell them back to the scrap yard for credit and grab some more. Works just as well as any of the fancy steel plates you can buy.