If your not familiar with cutting and lapidary, this onyx job is
probably a good opportunity to find a good cutter inlay shop to do it
for you. As I recall the last hole I had drilled in onyx (3 years
ago, or so) was about $10. You should be able to find a suitable
shape available without cutting from scratch. Most gem suppliers have
cutting services available.
I will make one suggestion that may make it turn out nicely. Set your
stone in your tube and bezel. Send it off, or deliver with the onyx
to the lapidary person. Tell them how deep, and where you want it. A
good cutter will fit your bezel to the stone much easier than you
could fit the bezel and tube to the finished hole. When both pieces
are returned with a nice fit, you will need to cut the tubing with a
saw, and burnish against the back of the onyx. Make sure they cut a
couple troughs on the back of the stone to firm up the burnished
Ask around a bit, you have an opportunity to find a lapidary person
to save you time and hopefully do some nice work for you. Good luck!