The pinhole diameter for a twelve foot (144 inch) focal length, according to the online calculator, is 2.55mm. In inch size, the diameter is .100" (1/10 inch).
Much smaller than a half inch.
Soda can material will work fine for a hole that size. Flatten it well. Or get some thin brass at a hobby shop. To make it easy to mount, you can drill it, then glue the material to a piece of wood or metal (like a washer, for example) with a larger hole.
A 2.5mm drill is undersize by only .001 inch. In inch size, a #39 drill (.0995") is even closer.
Use whichever one you can find. The Dremel section at a hobby shop or possibly even a hardware store might be your best bet to find individual drill bits in that small a size.
You could clamp a piece of sheet metal between two pieces of plywood or other flat stock and drill through the whole sandwich to minimize tearing up the edges of the hole. (Drilling through relatively thin metal that isn't well anchored can get quite exciting!)
You could make a larger opening and add a frame to put aperture cards in it.
Types like these: http://re-inventedphotoequip.com/Sit...gurations.html
Maybe Reinhold (the lens builder from this website) can make you a card with a smaller aperture.
Or just make one yourself from wood with a shim in it.
This way you can adjust your pinhole size easily if you want to make the focal distance smaller.
Projecting the whole image on the back wall or making a print with photo paper at 1 or 2 feet from the pinhole or ...
You could even add a red filter to position the photo paper before making the actual print.
Use Duct tape on the outside as a shutter ;-)
I turned my classroom into a camera obscura one year. I used a piece of copper flashing for the hole, drilled it between two pieces of scrap boards as has been mentions then ran a debur tool around the inside. I did not have the right size hole but it was close enough. We sat in the dark for a while and as the kids' eyes got accustomed to the lack of light they got more and more excited as they watched cars drive past upside down and backwards.
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.