Actually, I think I found his answer:
"Make sure to purchase a chassis (sometimes called radio) punch to make this hole. Conduit punches are also available but the punch size does not actually match the size of the hole it makes as the punch size is actually the NPT (national pipe thread) pipe size - just to make things confusing! When in doubt double check what size hole the punch will actually make before using it! The punch you want is the GreenLee 1-1/4" chassis/radio punch that makes a hole that is 1-1/4" (1.25") in diameter" - p 43 of the e-book.
The conduit ones still seem much more readily available, I wonder if they will work, if i can match the actual hole size.