If you are going to use this design, tilt the plates forward at 15 degrees so that the lead ricochettes downward, into the ground for safety. If it sprays upward, or out to the sides, it can do a lot of damage a long way away. I have seen lead ricochette off steel bang plates come all the way back to the 100 yard firing line. You will need very thick steel to stand up to to even light loads from your .54. The closer you shoot at such targets the faster the target will bowl, and finally punch through, as the repeated strikes will first work harden the surface, and then make them brittle.
I have a friend who makes these kinds of targets for companies to sell, and If you are truly interested I can put you in touch with him directly. I am not sure you will save money, but he can pick the right steel for your targets and build them so they will be safe.
personally, after years of building and shooting at bang plate targets of all sizes and shapes, and with all kinds of guns, I prefer to have the targets hand from chains, with the chains mounted to the back and about 25% of the way down from the top edge of the plate. That puts the correct tilt to the target, to deflect lead downward into the ground. After a couple of years, you can take a shovel and pick up the majority of the lead from the ground and use it to cast new balls. Swingers are nice when you are shooting a .22 rifle that is semi-automatic and offers you rapid reloading. Those kind of impact plate swingers ARE available from the sources you mention, and they do work. I have used a tree to teach kids to shoot .22 handguns at short distances, and they had a ball. Again, you don't want the bullets bouncing back at the shooters. Tilt those plates down.
