These are inexpensive, very easy to make, and only take a few minutes for each one. They work very well, especially with all the orchids I have in my greenhouse.
I have the bottom tray filled with water. I also have added some 'mosquito bits' to the water to keep mosquito's out. The humidity stays about 30% on the low end and up to 80% or so on the high end even though they're in a greenhouse and it gets pretty hot in there. They also help keep the temperature down around the orchids.