I used both a hole saw and a rotozip bit in a dremel. Both tools worked equally well. The hole saw makes a perfectly round hole, but I'd be concerned about it breaking out. The rotozip tool is very controlable and will last for maybe 3' of cutting before it burns out.

If you want to go the ROTO zip way you can cut half way through with the Hole saw then the rest of the way with the ROTO Zip. This should leave you with a round hole

Jon

I finally tackled this job last night. I got a 90 degree attachment for my dremel an was able to cut from the top. I used a tile cutting bit. Worked great! It cut like a hot knife through butter with no chipping at all.