I have a small Ethernet switch that I'd like to mount in the entrance corner of my crawl space, where there's plenty of room to stand and electricity.
I'm having a hard time finding one, though. Can someone recommend something for this?
If your equipment generates enough heat that you have to vent it to prevent overheating, it's not going to suffer from condensation.
If your equipment is in danger of condensation, put it in a sealed box, it doesn't need venting.
What you want to do, may not be the best idea. Typically networking equipment (and more generally electronic equipment), is best installed in a conditioned space. Ideally, you'll want a clean, climate controlled location. A crawl space generally does not fit the bill, and may be the exact opposite of the ideal location.
Some of the features of the enclosure may directly contradict each other in an environment like this, making it difficult and/or expensive to find a solution. For example. The enclosure should be enclosed to prevent dust, dirt, debris, moisture, and bugs from entering. However, you'll need ventilation to prevent overheating.
In a conditioned room, you can typically be sure the equipment will only be exposed to moderate amounts of dust, dirt, moisture, and bugs. In an unconditioned space; such as a crawl space, all bets are off. There could be dust and water everywhere, extreme temperature swings, etc. in a crawl space.
It's possible that you're having such difficulty finding an enclosure, because the best solution is to not install the device in this location. Maybe others have attempted this, and concluded that the easiest solution was to install the equipment somewhere else. Sometimes "The only winning move is not to play."