If you must use a template, you can get the Stew-mac one, locate it for the left side, cut the left side, then reposition it on the right side, etc. I haven't seen the Stew-mac one but I would assume it's symetrical. That could be your only problem, if it was slimmer on the bass side, because that's how the Schaller & some Floyd baseplates are cut. If you call, the Stew-mac reps should be able to answer this one for you. Otherwise you can make your own template from acrylic.
I just measure and tape it off once the trem is in place, and route it. I prefer the back of the cavity (under the fine tuner area) to angle inwards, making a trapezoid instead of a rectangle. The edge angles that way, and I think it looks cool.