I agree with E. van Hoegh. (It looks like my original guess was on the right track.)
The ideal pinhole is a perfectly round hole in an infinitesimally thin opaque sheet; e.g., a knife edge around the hole. All the supporting structure needs to be totally out of the light path.
I have more than you may want to see on line of the 4x5 'box camera' I made last year. The holes the pinhole plate mounts against were at least 3/8 of an inch in diameter. In addition, the surfaces away from the pinhole plate were counter bored at about 7/8 inch diameter to about half the thickness of the 1/4 inch plywood pieces. That camera uses a dimple-and-sand pinhole in 0.001 inch brass shim stock (and may be the best I've made so far).