Using PinholeDesigner with a user constant of 1.4 (determined by empirical testing for optimum on-axis sharpness) I get a pinhole diameter of about 2mm. We rarely agree on the perfect formula for determining pinhole diameter; that's part of the magic of pinhole photography. A slightly larger pinhole will favor sharpness towards the image corners. With such a large project, a little experimenting is wise. Pinholes that large are easily drilled between sheets of hardboard, hardwood, or even plywood, as Dave recommends. A larger drill or a countersink can debur the relatively thick material that is appropriate for such a long focal length. I used the end of an ordinary tin can for the pinhole in a solar eclipse camera about 24 feet long.