While the initial cardboard-box prototype has been quite a success, clearly for long-term reliability, waterproof enclosures and connections will be required. I have ordered samples of several connectors to evaluate their fitness for this project.
To connect the reed switch sensor, I decided to try out the EDAC 560. Rated IP67, this should provide more than sufficient protection from the environment -- assuming correct crimping. These are only available in a wire-to-wire variant, and they have a disadvantage: it is not really possible to apply strain relief for the individual wires near the connector.
The second sample is the Weipu SP13, a wire-to-wire, soldered variant. Its clones are abundant on sites such as Aliexpress, but I decided to try the original due to its good availability. This is a 4-pin version, which I wanted to use for connecting the front I2C bus (which carries the OLED screen and, in the future, buttons via a port expander). It feels very well-made, however, as expected, it is quite large, so I will not use it at the moment. Instead, I will try to get my hands on a pair of Weipu's miniature SA6 series (these are 6mm aluminium connectors)
Finally, a pair of 14-pin TE AMPSEAL connectors (1-776262-1 + 776273-1). And boy, are they massive. A wire-to-panel combination, these were candidates for the connection between the "ECU" and the cable harness, but I think I will instead go for the more compact SP21 (21mm round plastic connector). I will need about 10 pins for version 2 of the project, so the AMPSEAL would be an unnecessary overkill (the only smaller variant is 8 pins).