Hi Pascual, Kirabi,
Not sure if you could make this work but maybe you could find a 12mm diameter coach bolt and cut the shaft down to plug the hole. The carriage bolt should have a smooth slightly domed head that you might be able to sand down and polish it to better match the hilt material. I was thinking of a bolt something like this made of aluminium:
And if the shaft thread doesn't continue to the head of the bolt you can use a washer to cinch/torque the nut tighter.
I think most hardware stores or online vendors should carry something similar to this bolt.
Hope this gives you some ideas.
For Tyeth