There are a few ways, so it’s hard to know where to start. Here are some of my first thoughts:
- Find a product that is IP68 and tear it apart. Maybe a GoPro is, but you can definitely find an IP68 project box at your local electronics supply store. It will look like this:
Notice that the joint surface is planar and the fasteners are outside of the gasket. Non-planar joining surfaces are a pain to seal properly, especially at IP68.
- Consider welding and sealing with epoxy.
For welding, search “ultrasonic welding”. There are many guides available and your plastics supplier can help you out on the final design details. It’s widely available at both North Americana and Asian factories.
Sealing with epoxy is also widely available. You’ll find this done in a lot of electronics. Things like relays are often epoxy sealed. It makes the product nearly impossible to take apart (which may or may not be a benefit to your design), and resistant to vibration as well as water tight. It looks like this: