Not sure about where you are but from most industrial suppliers here you could get a hole saw to make that cut for about AU$35. An arbor would set you back a little more but you would get a perfect hole and the fitting instructions or the factory would be able to tell you exactly what size.
My last two 4wd's (Ford Courier & GU Nissan Patrol) I've fitted my own snorkels to them. The dealers or 4wd joints charge about $400 for what is a simple job.
It's pretty scary dropping a 100mm plus (4" +) diameter holesaw through your shiny quarter panel (guard). On the Patrol I had to do the outer panel and the inner guard.
I know how you felt cutting your AI hull.
Dave