Sir, hope your well brother,
Depending on the price of using the waterjet, you might be in it for $100 just to cut that one target, plus the cost of the bracket/etc on the back to make it a flapper.
A good way to get an effective flapper, is to cut a target with a little leg on it, and weld a thick and strong tube on the back, then simply slide the target leg down inside the tube. Mount it at a slight angle, and you would be good to go!
Here is a pic of the back of one of ours. Even though this is a double, its pretty darn close.
Look at the tube in the middle, its slightly canted,
But the problem is really that when you want a 10 inch round flapper, you really need an 11 inch round flapper target, just to cover the sides. And the machine, will take a little away, Even if its a 1/16" on each side. There would still be a 1/8" gap if you cut that target and used the circle in the middle as a flapper. Unless you were to buy another target to make a flapper. BUT, then you count the cost to cut everything, and buy another target for the flapper, it gets expensive,
might as well but another flapper target for a shade over $200 and keep your other target in one piece.