Don:
I would make that connection by drilling and taping 1/2-20 providing a lug and 2- 1/4 20 hex nuts to actually lock the connection tight to the threads and enclosure.
On the project we use a GE Mini Load Center with a 480/120/208 transformer with a 12 circuit panel all in one enclosure. That is exactly the way GE provides a ground attachment. That is less one hex nut. They only use one.
