If that Mustang is still humming after shielding, check the continuity of the shielding. Does it go away when you touch the strings/bridge? I shielded my JMJ but still had some hum, I found I wasn’t getting continuity from the pickup rout to the control rout. Ran an extra piece of copper tape from the pickup rout over the top of the body so it made contact with the control plate - all peachy.
I like those pickguards.