If you're wanting to buy, a google search should be able to find something. If you're looking to custom make, I don't know of any particular plans, but again, google might be your friend. You'll probably want to look at something like AR500 alloy steel plates. A google search should be able to find some of those, too. Then you should be able to just study the way some of the small bore resetting targets are made, scale everything up and be good. Sorry I wasn't more helpful, but maybe this will keep the thread up long enough to get someone who can help find exactly what you're looking for.