Wood hole - Water damage

I would remove some of the rotted wood to see if any mold is in it. If no mold i would vacuum out any loose wood and use a thinned epoxy and fill the hole. The epoxy can be thinned with acetone or you can use an off the shelf product like water glass or git rot as bumpy has mentioned. You want the resin to flow into any separated layers or wood, that is why you want to thin it with a solvent. The solvent will flash of quickly during the curing process. Once dry you can cover it with caulk or tape. If you have mold or the damage is greater than you described, the repair would be different. Let us know want you find.