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Bong stems come in various diameters and lengths. You may have to do some checking to find the right fit for the bottle you're using. If you're unsure about the size, it might be smart to buy a couple different stems and see what works best.

An average glass bong stem runs between $5 and $10, depending on where you look.

If you do have to buy the bowl and stem separately, make sure the bowl is the appropriate size for the stem (the measurements should be displayed prominently on the package) and attach the two by sliding the bowl onto the thicker end of the stem.

Purchase a glass stem or set of stems to fit the hole you drilled. Check to see if you have a tobacco or "head" shop in your area, as stems are typically sold there, or buy one online and have it delivered. A bong stem sometimes includes a bowl; if not, you'll need to snag one of these too. As the hole you drilled was 1/2" in diameter, you should look for a stem with roughly the same width, and it should be long enough to reach to the midpoint of the bottle so that the bottom of the stem is partially submerged once you fill the bottle with water.