Adding a Shutter
It's possible just to use a piece of tape across the pinhole to act as a shutter, but a sliding shutter can be easier to use. Cut two pieces of thin card, a square about 32mm, and a rectangle about 25mm x 40mm. In the square piece, cut out a 6mm square in the centre.
Place some black tape on one side of the rectangular piece to help prevent light leaks.
Place the square piece over the pinhole and tape down three sides, leaving a gap in the top into which the rectangular shutter card can slide
Check that the shutter can be pushed down to fully cover the pinhole.