The handiest simple tool in the world (that most people don't own) - a sheet-metal worker's T-reamer.

This utterly simple no-working-parts hand tool will easily, smoothly, safely, and precisely enlarge round holes (and keep them perfectly round) in any rigid, non-brittle material (i.e. not rubber or leather, not glass, but almost anything else). I discovered this tool by accident when I was twelve years old and have owned one, more or less, ever since. When I lose one, I have to buy another. Costs a few bucks, and, with regular household use, never wears out. Try it. You'll like it. Don't get marooned on a desert island without one.

-- William Gibson

T-Handle Taper Reamer
$4
Amazon