As part of my pinhole camera making, I want to make one of those simple lensless "wire" viewfinders like the ones you see on the old press cameras. Basically, the ones that are just a little frame in the front with another one in the back that you look through and line up sort of like the iron sights on some sort of gun.

I realize that I probably just need to do some trigonometry to match the size and distance of the "sights" to the angle of view of the camera, and I know I won't easily be able to come up with any sort of parallax correction scheme (if pinholes have that problem), but anything better than guessing would be helpful for aligning a pinhole camera.

Does anyone have recommendations on how to make one of these?