I built one pinhole camera out of matt board, stappled together and black electrical tape over a couple of the seams. I have left paper in it for months and it's been fine. If you have an enlarger handy, use that to measure your pinhole accurately. Place the pinhole in the enlarger and project the hole on the baseboard. Measure the hole (you can also expose it onto some paper if you like to make it easier to measure and to examine for roundness and any daggy edges). If your enlarger column has the magnification on the column, read that and calc your pinhole size (measured circle / mag ratio) or work out the mag ratio by measuring your neg carrier (empty) and the projected rectangle and calculating (projected width / neg carrier hole width)