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.