I took a slightly different approach from Matthew. Knowing the angle of acceptance of my Ilford Titan, I printed a similar triangle on a transparent sheet. I placed the apex at the pinhole to get an idea of what the field would be. It oddly enough worked quite well. I then went to a hardware store and bought some metal rods and a thin metal sheet. I soldered the rods to the sheet using the plastic as a template and made a clamping device so I could attach and remove my viewfinder for either a vertical or horizontal format. I painted it black so it looks official. And although it may not be completely mathematically correct it works pretty well. After using it for a few shots I can eyeball rather closely and whatever it is off so be it -- that's part of the lure of pinhole any way.
The materials cost under $10USD and it took about an hour to make.