Turn your salad bowl upside down over your water cooler. The bowl should sit easily on the rim of the water cooler without slipping down inside. Bowls with a wide lip around the edge work best.
Measure around the inside edge of your bowl. Cut a piece of rubber tubing to match this length with your utility knife. Trim the ends to 45-degree angles. Make sure one angle points up and one points down. Bend the tubing into a circle; the angles should match up and create a flush joint.
Run a bead of silicone caulk around the inner edge of your bowl. Press the rubber tubing into place along the bead, making sure the angled ends meet in a flush joint. Let the caulk cure overnight.
Fill a mason jar with tap or rain water and set it down in the center of your water cooler. Fit the bowl into place on top of the cooler and set the cooler in a sunny place. The sun will evaporate the water from the mason jar. The water will condense against the inside of the bowl and run down the inside of the water cooler. After a few hours, the mason jar should be empty except for any impurities that are left.