Profile Design compression plugs on eBay (or Treefort, Niagara, etc.) go down to 19.6mm. Can't vouch for quality, but have used one of their stems in the past. Or maybe an older Cinelli 1".

John

Edit added: I would think a 22mm minimum would be for a 1-1/8" steerer considering a 1" steerer has a 25.4mm OD. 1.7mm wall thickness seems a bit thin for a carbon steering tube.