I think that with piezo transducers mounted round the edge of the target at 120 degrees to detect the shock waves should work as the shockwave propagation should be pretty much linear to a high order.And looking on a scope you can see pretty sharp edges using a wooden board and hitting it with a ball bearing from a catapult.
Thats my plan anyway and I am sticking to it.
I dont want to put anything more than I have to on the target and metal plates would just add to the confusion i think.
Also I am not at home to non-linear effects thank you.