All of the junction boxes I found on here either had insanely overkill 4mm wall thicknesses and screw-mounted lids, or were too basic for my liking - so I designed my own!
This one is modeled after an existing off-the-shelf junction box with a wall thickness of only two millimeters and has a lid that simply clips on instead of requiring mounting hardware. It has drilling guides on all four sides for the cable inlets and should be dust- and slightly splash-resistant (provided the cable is a tight fit in the holes you drilled; I would rate it as IP43 or IP44). I'd still only use it indoors or under a roof though.

WARNING: THIS JUNCTION BOX DOES NOT COMPLY WITH ANY OFFICIAL SAFETY STANDARDS AND/OR REGULATIONS AND SHOULD THEREFORE ONLY BE USED IN LOW CURRENT / LOW VOLTAGE APPLICATIONS! ANY USE IN HIGHER POWER APPLICATIONS MAY RESULT IN DAMAGED/DESTROYED PROPERTY AND/OR LOSS OF LIFE. I DO NOT ACCEPT RESPONSIBILITY FOR ANY DAMAGES RESULTING FROM IMPROPER HANDLING OF MAINS VOLTAGE!

So yeah, use at your own risk - junction boxes are usually made from fire-retardant plastic, which is why using this for anything more than a couple watts of lighting is a bad idea in case there were to be a bad connection. In my case, I made it for a 12V solar system with 200W flowing at best - still technically more than enough to cause a fire, but since that's the absolute maximum my panels could theoretically put out, I figured the risk is relatively minimal.