I installed a IP security camera over the weekend, and though it says "outdoor" on the box, apparently that just applies to the camera itself, because the wiring pigtail had a little tag on it that said "Please weatherproof cable ends if installing outside!". So I checked if I could find maybe a tiny version of this: https://www.amazon.com/Farm-Innovators-CC-2-Connect-Water-Tight/dp/B002MHFXS8 but I couldn't, so I decided to build my own based on one of the existing screw libraries on here.
I'm not sure how weatherproof it actually will be, I may open it up and add some silicone sealer in the spring. It was quite a pain to thread on though (possibly because of warping), so I think it has the added benefit of making it harder for someone to just unplug the camera.
UPDATE: Version 2 was designed based on suggestions from @frankvdh. It is meant to be hung vertically, with the cables both entering and exiting from the bottom. The idea is to simultaneously give water fewer ways in and to let gravity take water out if it does still happen to get in. I haven't printed it yet, but the theory seems solid.
You can now choose between the original and V2 for the cable cover, both are designed to work with the same "nut". All parts can be printed without support, though you may want to add helper discs.
Usually I would print this sort of thing in ABS for durability, but I was more concerned with just getting this connection covered on the warmest day for working outside in the foreseeable future (which was still too cold to be fiddling with printing ABS) so I went with PLA. This is probably not a shortcut you should take if you will be placing this thing somewhere that gets direct sunlight.
This was designed around the two specific USB connectors I would be fitting inside it, which had 4mm cables, a width of 11mm, thickness of 8mm, and connected length of 58mm including the strain relief bushings. I added recesses on either side so you could maybe slip some o-rings onto the cables or just pack the gap with sealer.
To assemble, slide the nut over one cable (unless one of the cable is still disconnected on the other side), connect the micro USB cables, slide cover 2 over the connection, add whatever sealants you planned on using, slide cover 1 over cover 2, and then thread the nut over the whole thing (you may need slip joint pliers to crank it to the end).