I assume this will be one of the first items you are thinking of selling for the Steem online store that you will be setting up? Not quite sure how you would make it water proof if the body is made from metal or wood as I was thinking of a rubber type of seal but then the temperature fluctuations would weaken the material.
I had a look at some of the waterproof usb drives to see how they were designed as well: https://www.toughgadget.com/best-rugged-waterproof-usb-flash-drives/
Wow thanks a lot for the link and I also think the cap might have to be screwed on in order to really make it waterproof - it's definitely possible - just a price question I think
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?) :)