Thanks all for the help.

I guess the lathe would have to be my first choice. I'll be heading to my grandparent's house for the holidays and my grandfather has a massive lathe that should easily do the trick.

Then again, the hole saw option sounds pretty easy, especially since, as mentioned, the rim around the hole provides a great guide. That was my first thought but I assumed that it would be to heavy a job for the blade.

The hole saw option also doesn't require taking the tires off and then remounting them, which is a bonus and, frankly, I could do it tomorrow. So I'll consider my options and post a photo or two when I'm done.
Stay tuned...

-Matt