To get back to the original question . . .
Bob's method works for a round speaker. But the round speakers available today are vastly inferior to the "cell phone" type rectangular speakers available from CUI, Soberton, PUI and others (buy from Digikey or Mouser). For the Kato E, F, PA, etc, use an 11x15 rectangular mounted at the rear of the chassis in its own enclosure. And just to reinforce the message: DO NOT USE A DRILL PRESS TO TRY TO CUT A RECTANGULAR HOLE UNDER ANY CIRCUMSTANCES.
Instead, the fastest way to do this for something like a Kato E or PA or F is what Peteski does: mark the slab of metal you want to cut in the rear vertically and horizontally; use a saw with a metal cutting blade to make those cuts, remove the slab, and then if you need to finish the cut, use a Dremel to do the finish work. BTW, I often use a thin cut-off disc in a Dremel to do a very light initial vertical and horizontal cut (sort of a tiny "ditch") to help guide the saw blade. And buy a machinist's vise. It is useful for all sorts of things, not just this.
John C.
