I've used the 4-11 (or 4") trick many times. Whenever possible, I secure them with pop rivets. When you are adding a circuit to 30-year-old equipment that might be 400A or more, and a full upgrade is not justifiable, and you just want it to pass...
This method meets Code, and is lots better than calling around to a dozen supply houses looking for this or that proprietary filler plate.
BTW, I usually secure them from the back of the deadfront. It looks better that way.
