There are some instructions out there on how to make your own pinhole 'lens'. You actually do not drill a small hole into the spare body cap as that is too thick. To get 'good' resolution with a pinhole it needs to be thin, perfectly round and obviously small, around 0.1 - 0.5 mm. This is best achieved by cutting up a soda tin, flattening out the metal foil from the side wall, putting it on a cutting mat and pinch a small hole with a sewing needle. Then drill a 4 - 6mm hole into the body cap lens and tape the metal foil with the pin hole behind. Make sure that it is absolutely central. I taped a cross-hair over the front for alignment. - I have only used mine in good weather and not near the sea for the reasons you have mentioned. Oh, and if you have extension tubes, you can use those with the pinhole to achieve a longer focal length (and exposure time ...).
There are some instructions out there on how to make your own pinhole 'lens'. You actually do not drill a small hole into the spare body cap as that is too thick. To get 'good' resolution with a pinhole it needs to be thin, perfectly round and obviously small, around 0.1 - 0.5 mm. This is best achieved by cutting up a soda tin, flattening out the metal foil from the side wall, putting it on a cutting mat and pinch a small hole with a sewing needle. Then drill a 4 - 6mm hole into the body cap lens and tape the metal foil with the pin hole behind. Make sure that it is absolutely central. I taped a cross-hair over the front for alignment. - I have only used mine in good weather and not near the sea for the reasons you have mentioned. Oh, and if you have extension tubes, you can use those with the pinhole to achieve a longer focal length (and exposure time ...).