I assume you're using a diamond-tipped drill bit for the glass. If so, go in perpendicular to the surface of the glass. When Drilling into glass, it's hard enough to get the bit to not slip; attempting to drill at an angle will make that damn near impossible.

Also, head shops that sell glass bowls will probably also have rubber gaskets to go along with the slides. Just ask if they have such a thing.

Also, a 1/2" diameter hole with a gasket fits a relatively standard size slide-- I want to say 9mm, but don't hold me to that.