[folivier]

I've had to do this before and usually try to attach a piece of plywood so the pilot drill can get a bite for the hole saw to start.
This looks like a good solution, use a hole saw the same size as the existing hole as a guide with the larger hole saw mounted above it:

[Fulltiming]

Great idea Forest.

[folivier]

Wish I could say it was my idea but truthfully I stole (borrowed?) it from a boating forum.

[HoosierDaddy]

Great idea!! Thanks for sharing.