If you are not in a rush let me see what I can do as I have some cork at home that I brought from Portugal a couple of months ago.
The internal bore of 5mm is not a problem as 5mm drills are pretty standard. I just need to find a 9mm hollow punch to punch out the initial cork slice before drilling the inner hole.
I am also seeing my brother in a couple of weeks and he might also have some ideas.
Otherwise you might want to start looking for 9mm o'rings and silicone grease.
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