Losing a bung from your kayak is a real pain. Nothing is child proof – hence why the little bit on the bottom of a kayak bung designed to stop you losing it is often broken on centre boats!
If you carry a couple of corks in your boat repair kit (along with some spare ratchet strips, cable ties and hot glue sticks…) they should stay dry and ready to use. Rotten cork wouldn’t be much help! Push/twist it in to the bung hole and cover with gaffa tape to see you right to the take out.