Cutting inspection plate holes: Here is one way to cut (perhaps scribe is a better choice of words) a nice inspection circle in your wing skin. I learned this when building (but not completing) a BD-5. The method is very simple.
Take one of your covers and secure it to the wing skin with some double back tape, hot glue or what ever you can come up with, so that it is held firmly to the skin. Then, simply start scribing around it with a very small and sharp scribe. After just a very short time, the piece will fall out. File OD & ID and you are ready to install the back up ring per the S-H manual.
Installing inspection holes in bottom wing skin: To ensure a perfect fit, install the nut plates on the doublers first, then screw or cleco the covers to the doublers. Then fit the whole unit to the holes in the wing and hold in place with tape or a helper. Drill the rivet holes clecoing as you go. This insures a perfect fit for the hole, vs. trying to eyeball the doubler placement.