A lot of terrarium builds I've seen have false bottoms created from eggcrate or other similar material that lifts your clay pellets and earth medium (sphag or cocunet husk,etc...) off of the bottom and away from the water resevoir. I personally built mine using eggcrate, cut it a little bit in so there was an inch around each side, and filled that in with pea gravel so you could not see the false bottom.
The one issue I see is that water stagnates, and you will need to be sure it's continually moving. My terrarium was only 1ft x 1ft on the bottom, and I have a waterfall feature so the pump keeps the water moving.
One note about ultrasonic foggers though. They tend to splash water all over the place. I tried one for a long time but everything just kept getting wet around it and the water resevoir lost water very quickly. If your orchids are sensitive to water splash, you will want to make sure they are far away from the danger or examine other means of fogging.
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