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Maybe if you create a cap mechanism like a pen cap over rubber fittings that would keep it nice and tightly sealed? The USB drives seem to just use the friction of the rubber to keep a nice seal and keep the water out.

I think there are a lot of people keen in the DIY/maker side of things on Steemit. Have you seen @thatsweeneyguy's project to create a physical makerspace on @thesteemhouse and the people who are promoting posts relating to this area on @steemmakers? I think if we can add a shop to connect all three of these ideas together that would be really great! Something for you to think about in Sweden (I assume you are about to leave China?) :)