If the guitar is hollow, you need to feather the hole, ie grind or sand a slope to the edges of the hole. Then, matching the grain, glue with epoxy resin a plug into the hole from the same, or similar wood. Sand smooth, stain and varnish. If the guitar is solid, use plastic wood from B&Q or similar.
... or you could just pay someone to stand next to you when you play and plug it with their ...finger?
Nick
July 2005