no need to guess the size of the hole, just measure it. The ID of the hole will vary with the wall thickness of the tube, even if the OD is known.
There are, of course, any number of round things that will fit into the hole. But the problem is that the top is sloped, so any normal thing will leave an exposed edge, right in the worst place.
If you cannot get the right thing, I agree with a post upthread; try a cork; jam it in place then whittle it so that it protects you from sharp edges whilst not sticking up too far (which will lead to it being damaged or displaced). BTW if the cork is not a tight fit, you can wind a few turns of tape around it until it is.
cheers