I always draw a big blob of black over the area with a permanent black marker and let it dry, and then stick a few layers of electrical/medical tape in the center. Then I take a 2-pointed compass (it's not technically a compass, is it? Some cartographics tool, anyways. I'm sure a compass would work as well), plant a point at my midpoint where the tape is, and scrape the circle. The marker gets scraped off where you need to drill and you have an easy guide to follow. I definitely find that even permanent marker doesn't hold that well so it's nice to have a pretty well defined circle if things go missing.
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