Often I find myself needing to join/split cables in a situation where its going to be outdoors in full on rainy weather.
I have tried using the common cable joiner enclosures like these:
But every single time, they all fail due to water ingress, no matter how much the seals are torqued down. It could be my shitty luck for sure, but after this happening enough times I’ve moved to just epoxy encasing joins.
Method
My go to method is to model up an open sided box enclosure with a hole for each cable coming in. Each hole is undersized by around 0.2mm on the diameter to ensure cable is a press fit into the plastic. This makes epoxy easier; since then it cant leak out around the cable.
I then 3D print this casing with a 2mm wall and 10% infill. Often on a fast setting as quality doesnt matter at all.
I then press the cables through the holes in the wall, and solder the connections. Noting that as this will be epoxy filled, you dont need insulation to stop the connections touching so long as they do not touch (i.e. epoxy will fill the gap later).
Once the wires are soldered together to make all the connections, slowly fill the cavity with UV resin. I find that doing this in layers and giving it a minute to settle before hardening helps prevent bubbles or bonding issues.
Once the entire cavity is filled and reasonably resin hardened, you are done.
As the join will exist outside, any unhardened resin will set within the next day.
I have had this an earlier copy of this particular join outside in full sun, for over a year with zero issues.