So, the bowl won't work w/ your grommet, right? Either get a smaller/thinner grommet or use some damp papertowel/newpaper and make your own grommet.
I have several narrow mouthed bowls and I end up using this trick with them. If it's still wobbly, wrap some foil around the bowl to give it some extra support.
And you think 15-20 grams is large? LOL - you've never seen a phunnel? I could pack an entire 50g box of tobacco into one with room to spare! Most of the bowls I currently own use atleast 20g-35g, if not more. But I LOVE to smoke ATLEAST 1hr-20min to 3 hours off a single session.
Good luck... if you file the bowl down, take care - you may weaken it too much and it may just break.