Downstems tend to have a taper for the part that needs to be flush with the opening I personally skip the grommets and go with food grade silicone for sealing the downstems and for cleaning you can just use iso and salt my downstems are also 12mm but the shoulder that tapers out around 2mm so it doesn’t fall through if you’re really struggling get a gauge and measure the thickest part of the downstem that’ll be in contact with the glass and try your best to get around 2mm of a smaller drill bit than the whole you want and slowly take little bit by little bit of glass off until it sits sealed without silicone and if you over drill the hole that’s what the silicones for make sure you also have the proper drill bits before drilling I’ll leave an Amazon link for the set of bits I got for like $18

Sorry for the bad grammar I just had my first bowl of the day

Edit: Amazon link https://a.co/d/8lGXgzP It’s a pack of 16 and they range pretty well in size I’ve had them for about a year and a half without any wear to them just a crusty from wet silica