I wonder if you can use an olla pot. I know this is used mostly if buried into the soil. What if you get the biggest olla pot you can find, fill it with water and put it right under your orchids..maybe the evaporation as it seeps out of the pot can provide the humidity you seek. But then again, you still have to fill it up with water, pretty sure evaporation/water leakage will be very fast and much more work to do in the end.
You would be better off with a spray misting system.
