Nice job. I have a buddy that does this and I called him to ask his way and he said he used a Hole bit like yours. The only thing he did different is to pack dish towels inside for pressure. But apparently that isn't needed as yours look impeccable.
You can also cut em with a low speed drilling with water. The water will wash the dust out and the slower the drill bit is moving the less rotational tension you have on the clay pot.