Excellent looking guide to waterproofing electronics, from people who have done it for underwater sensors: Waterproofing electronics.

Conformal Coating

Metal can corrode, but provides RF screening, and may be stronger if it could get bashed. RF screening is generally good, especially when you are putting it beside other radios up a mast, but bad if you want to talk to small WiFi boards that are inside it. :) Also not suitable for putting a lightning detector inside.

This is a HAMMOND 1590WPFL, which I got to try.

I did not consider when ordering this box that it has internal ribs. This is a problem for forming a watertight seal with connectors that want to do that on the inside. I ground an area flat around the hole for the NMEA-2000 connector, and applied plenty of butyl sealant tape.

The N type connector in the foreground is designed to seal on the outside. I choose this because it seemed like a better idea. This kind is harder to find. I thought the sealing o-ring was barely thick enough though, so I added butyl here too.

This box type is also not that great, because the seal is a stick on rubber gasket that goes on the edge, without any flange. It looks like a box that was not designed to be waterproof, but the seal was an add-on. I'd get something better next time.

I also noticed now that the picture on Farnell is wrong. The datasheet is correct though.

It comes with zinc plated screws and rubber o-rings for them. This is not ideal - both because the screws will probably corrode themselves in salt water, and because of galvanic corrosion with the aluminium box. Marine stainless screws with a suitable compound would be much better.

Another detail is that the box is thinner at the bottom, which you need to allow for if measuring. Maybe better to design inserts / PCBs to the datasheet spec.

I also have a plastic box. It seems to be meant as a junction box, and has knockouts on the sides. These are far too large to be useful for my purposes, so they will just weaken the box. It has plastic fasteners to hold the lid on, which is good for corrosion, but no way to mount it without drilling a hole to the inside, or glueing something on. Some boxes like this do have a place to put mounting screws under the box fasteners, but this one doesn't.