Yea man the way i do it if you make the gravity bong out of a plastic bottle with a screw on top

Get a steak knife and stick it in the middle of a bottle cap. If you apply pressure and spin it around it'll start poking through then just rotate it really fast to grind the hole into a perfect cirlce. Then use a slide from your bong and stick it in the hole. Start with a small hole then keep checking to see if the slide will fit snug.
Gum works too