Boy I hate to quote myself!! I'll post the method that I used to locate the servo bay and install the hatch, since it worked out so well for me. Future builders may find it helpful.

I used the cover screwed to the rails to install the rails, as the instructions said. After sheeting was applied I gently cut out the bay to the edge of the rails and the ribs. At this point, I took four tee pins, and bent a 90 degree bend about 3/4" from the end. I poked the pins through the sheeting, from the inside of the bay, so that the sharp end was poking through the sheeting at the four corner holes of the rails. Then I just laid the cover over the pins, centered up the pins on the corner holes, and used the cover as a cutting guide. Dropped right in, lined up perfectly. All that worrying for nothing!

Pictures later. I'm on the wrong computer.