Copied this from MNR Discussion Group.com Great for Mirror Dinghy information
`The first thing I would do is enlarge the hole to make it a rectangle with 4 90 degree corners. If the original hole is roughly the size and shape of a boot I would think the most convenient thing would be to make it a rectangular not square.
Then around the edge of the hole and I would work from the outside in take out a layer or two of ply about one half inch wide all around the rectangle.
Now get a piece of ply the same thickness at that used for your hull. cut it the same size as the hold plus the width of the ply layers you removed. Now take appropriate layers off the patch to mesh as perfectly to the hole in the hull as possible. Try and cut the patch so the grain goes the same as the ply in the inside hull. You would do this for esthetic reasons. When you are convinced that the patch is a really good fit to the hole glue it in place. I would use west epoxy if you can get it in the UK. If not ask around for what ever equivalent epoxy you can get.
Once the patch is glued in place, make a paste of epoxy and microballon and butter it on the outside of the patch. If you do this well it should allow you to make the patch invisible after it is painted.
On the inside I would clean off the patch and surround wood and put a good epoxy coating over it.
This should get you started. Other people may give even better advice. My advice comes only from one experience on my boat and it worked out quite well for me.'
Mike R
July 2007