humidity sensor is on the unit itself (display portion), on the back near the bottom. I just lay mind on the substrate on the cool side. Cypress mulch retains some moisture, you may want to stick it an inch or so above the substrate. Get the 3M velcro tabs, or find a suction cup that will hold it up. Though my boa decided she doesn't like hers on the side of the tank, I think she comes out of her hide just to knock it down.
Edit: On another note, why cypress mulch? From what I have read about it, its prone to mold and should be replaced weekly (though that's if it's sprayed every day). My ball sheds fine on aspen.
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