I do the cutting pretty much the same way as you Admin. It goes pretty quickly with the dremel and sanding drum.
As for locating the starting point, I stick a pin through the plastic at about the location I think is close. I peek inside to see what part of the engine the pin is contacting. When I've got it just about where I want it, I slowly start opening the hole up, constantly test fitting, until I can see what I'm doing through the hole itself. From there, it's easy.