I did exactly the same thing when drilling my Coleman Extreme MLT for the thermothingy. Fortunately I need a separate 1/2" bulkhead for my sight glass anyway, so I found a fitting that will let me mount both to a stainless 1/2" T fitting and just enlarge the hole to fit a 1/2" nipple.
Carboy 1: Dry Cider batch 2
Carboy 2: Imperial Wit
Bottled/Tap-a-draft: Dry Cider, Tripel, Fat Tire Clone, Belgian Raspberry Ale, Patersbier, Double IPA
Planning: First all grain brew, new brew room, long draw 4 tap keg system/keezer