Is just water at ambient temperature.
The first containers was made a year ago. It provided a cheap and efficient solution.
For best results next step will be use cold water. But we not have tested that option yet.
Outside temp is 32c
https://imgur.com/P0wkYjEand behind the pads is 28c
https://imgur.com/r7OfrrWActually I'm more worried with that humidity increase, that is very important. If i were you, i wouldn't use the wetting thing. Lowering the outside temperature isn't as critical as that humidity. You will see more rust if you keep those containers at 72% in exchange for reducing a mere 4°C.
Let me tell you something, I saw a warehouse in a tropical climate, regular under roof is temp was 40°C, but with some thermal insulation, it came down to 30°C. No messing with an already high ambient humidity (which ironically can be brought down by putting some of the hot exhaust air back in).
To be blunt i think you are making a mistake in your containers, just worry about moving air, not cooling it down. But if anything, yes, just add a roof on top of the container, perhaps with enough space so that air can flow between. For added points, put some solar panels. Regular filters (non wet) are good for dust and such, those are fine. Of course i know there is a system that "self cleans" itself by using water, and as a side effect you get some cooling, but also humidity... Electronic equipment doesn't like too much humidity (or too little, your idea can be a blessing in dry places).