What size cave do you have? If it is small you may be able to do a passive setup with a bowl and sponges.
I am not positive, but I think the Perfect Cheese unit is what I call a 'swamp cooler' in that it pulls air through a water saturated mesh wick. I have heard that some people are happy with them but other says they they don't do the job for them But from my experience with swamp coolers having lived for awhile in Arizona, they are best effective in a dry air environment.
Another option is to use a portable ultrasonic humidifier with an inexpensive controller unit. I use one of these in a wine cooler along with a cheap reptile controller. If used in a small cave, the disadvantage is that the height of the bottle can limit how low your first shelf can be positioned. They also produce the most fog if the water source is warm. However, even at 55F, they are still effective enough.