I cut 3/4" marine plywood just slighty larger than the opening,then sanded until it was a snug fit using the handle of a hammer to lightly tap it in. Then covered it with mat and pure west marine epoxy. Dewaxed, sanded then another layer of mat and west marine epoxy with microfibers in it. Dewaxed, sanded and primed and painted.
Here is a link to pictures:

http://www.facebook.com/home.php?#/album.php?page=1&aid=2441&id=100000080723823