I made a pinhole lensboard for this weekends WPD by glueing 4 layers of mattboard together. Made two pieces 87mm square and the other two 100mm, stuck them together and it fits perfectly. Used a holesaw to make a decent sized hole in the middle and taped my aluminium can pinhole across the hole (I choose to tape it across the back as it looked better!!!!). To measure the pinhole I put it in a enlarger and project the hole onto a piece of paper to check it's round and to measure it. I then use the magnification ratio of the enlarger to calculate the pinhole size. Seems to work out close enough