Don't worry about it. If you're worried about the open hole use some chewing gum - if you are worried about the rubber bung use a telescopic magnet to get it out.
If you want to do some real damage drop an 11mm socket down the timing hole on the top of a Peugeot 405 diesel bell housing then drive for 100 miles.
In my defence I didnt know it had dropped into the bellhousing till I removed the wrecked the gearbox and diff and found it in there.
A rubber bung I wouldnt worry about.