You could use the cylindrical or conical metal burr bits, or if you have it, the side cutting bit that looks sort of like a drill.
I also have a dremel, but for job like this I'd just pull out a hand file. A hand file offer much more control, with less chance of cutting small gouges as you move along. Use a half round file, and if you can find it, a half mill (teeth are cut in one direction only) will leave smoother edges.
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Last edited by FBinNY; 01-28-12 at 09:45 PM.