I came up with this last night and it works awesome. Get a regular pipe screen you would put in the bottom of your bowl (I used brass because it's more flexible than the other metal ones) and put it IN the bottom of your downstem so it's not sticking out. It makes a noticeable difference; my bong is now way easier to pull and the smoke is slightly more cool. No need to splash out on an expensive glass downstem with holes in it EDIT: I guess it works because it stops the smoke coming through in one big bubble
such a sick idea cant believe no ones thought of that yet but there are glass diffuser stem adapters that go on bottom of normal stem for like £2