Basicly what I did was measure how much of a margin you have on all four sides where it scoops in and mark it out on the front bumper, try and go conservative at first and test fit it. It probably took me a couple hours to put mine in because I prefer to start small and then trim it with a knife.
But yeah your right a template would take the guess work out of it. But its good to know you would have to cut a crap ton out to cut too much. So just take your time!