I've recently done that also. Here's a few pics....

The first one, I made a filler plate, that gives a solid surface for things once you cut a hole....ALSO........the plate was cut and fit in BEFORE the hole was cut. I just made the plate fit nicely from within, and when satisified, wicked some CA in there to glue the plate in. You want to be careful to not get to much though. Go slowly with the CA, it will save your exterior. I just knew my ball-park for the hole, and made the plate to accomodate within reason.........