if they do it professionally, they will have a round jig and will router a series of curved lines where the speakers are, they might have a jig with a series of holes in in a circle shape, then just drill a series of holes
when i used to line vans for the local council, the guys with speakers in the rear would get a series of holes in a 9 " circle
the circular lines or holes both look good, i prefere the routered option myself though