For the 2011 Worldwide Pinhole Photography Day I made a new camera designed to use a 4x5 filmholder and made a pinhole with 1 mil brass shim stock. It worked even better than an earlier effort with a special lensboard for my old B&J 4x5 that used thicker stock. The dimple and wet sand method can yield a near knife edge if done very carefully. You can also make some iterations of gently twisting a needle point and sanding to improve the roundness and edge quality of the hole. I used a a magic liquid ("Blacken-It") used by model railroaders to blacken it which should have included any edge.

More than you wanted to know about the camera here.
And the 2011 WPPD results here.
The paper negative stuff needs some refinement, but the film negatives were pretty nice.

One of these days I need to work out a stand to use my B&J with an adapted Graflarger back to enlarge the stuff.