I’ve seen it mentioned here that a sieve full of coarse salt on top of a bucket should be excellent for lowering humidity in a passive cheap way.
Never tried it, and not sure if it was a sieve or cloth or some other setup.
Does anyone else remember?
Edit: found it.
To make a rock salt dehumidifier, you’ll need a large bag of rock salt (also sometimes referred to as sodium chloride) and two 5-gallon buckets. You should be able to find both at a hardware store.
Drill several small holes in one of the buckets along the sides and the bottom.
Put the bucket with the holes inside the other bucket.
Fill the bucket all the way up to the top with rock salt.
Place the bucket in the area of your home you’d like to dehumidify.
The bucket will begin to collect water, and that water will drip through the holes of the inside bucket.
The water will then collect inside the outer bucket, trapping excess moisture.
Source: 6 DIY Dehumidifier Options