dude just go to the head shop, and tell them you need the entire bowl/slide assembly including the grommet, then when you get home, make the hole the size of middle of the grommet...

i can't think how to explain this, ok there will be the hole cut outta the center, that the slide goes through, and if you hold that upright there will be a slit through the center going across, you have to make sure the outside rim of that has enough room to clear, because if you drill the hole just the size of the inside hole (that the slide goes through) when you go to put it all together, i garuantee it will break something, prolly the slide.

and this may sound weird but when you go to put it all together and you have the grommet already in the bong and you go to slide in the slide, make sure you lick the outside of it really good, so it slides in easily.

btw the slide is what the bowl goes in, incase you didn't know.. just trying to help you out.