If you own a Better Bottle...

I have no experience with a Better-Bottle. In fact, I had to look them up just now. :^) But I have extensive experience with using spade bits and I would suggest a hole saw instead. The center drill on a spade bit is tapered and enlarges as it gets close to the blades. In a thin material this can cause considerable "walking" of the bit and result in a hole that is not concentric. A hole saw uses a straight bit has a guide and I think you would achieve a much nicer hole.

Quixote

I have both types, and I have found that spades will walk on food grade buckets if I'm not careful. I might give the other a go. If my first cut is bad, the rest of the Better Bottle will become the experiment to see what does work.:)

John