The large hole for the HID projector can center right on the headlight bulb hole that was originally there. I just started tracing a circle around the projector and started out small so that I could slowly enlarge the hole to get it just right without any gaps.
Here is the size of the hole once I finished cutting and shaving it. I used a air powered die grinder with a diamond steel cutting bit. You need to use a high speed rotary tool of some kind because the reflective plastic is really brittle and can crack easily if you use a slow cutting motion. Now getting the projector to stay in one place is tricky. I used a piece of wire wrapped around the two screws above the hole to hold it in. If I spent more time I would use fiberglass or plastic weld material to make it really nice and sturdy... but the thick wire should do just fine.