I've also had some come through that had a wedge shaped piece of plastic with a split down the center that extended from the bottom of the guide, pushing up into the hole.
Personally, I'd just drill and tap it for a 5mm bolt, that way you can use a standard water bottle bolt that is cut down.
The hole is already larger than a braze-on bolt otherwise I would have done that. Good suggestion though. I like the plastic expansion bolt with pin going through it (like from auto application) even better!