You could try doing something like those old chicken watering stations, or primitive top offs.
just get a container, fill it with water (ideally distilled/RO, but I imagine whatever you normally use will be just fine), and invert it with the opening just below water level (You'll have to make some sort of stand, or liberal use of ducttape to hold it in place).
Once evaporation lowers the water level below the opening, air can get into the bottle, allowing water to flow out and refill the tank up to the opening.
If you have some big (~1 gallon) juice/milk/soda/whatever containers, two of those should have you covered, but even just 1 or 2 liters/quarts would help lessen the water loss. might want to put some plastic wrap around the area where you mount them, just to lessen evaporation where the opening is.
Won't be pretty, but if no one is there to look at it, it shouldn't matter much...