No idea how well it will work, but as to a plug-
go to a hardware store and poke around where they have the little boxes full of screws/bolts/nuts/whatever on the slide out trays.
Sometimes little neighborhood hardware stores have a much better selection then the big-box stores.
Anyways, oftentimes, there will be one tray that will have an assortment of rubber and cork stoppers. I think you should be able to find one the right size for your needs.
A more temporary/short term solution, you might be able to just place a wide rubber band over the hole (like the type they use for broccoli), or a section of bike inner tube, etc. -wrap it around the tube so it's taut, and then place it over the opening. wouldn't be too pretty though...
Last edited by lochaber; 10-04-2013 at 08:34 PM.
Reason: new thought