Portable Paper Wetting
You don't need a tray to wet watercolour paper for stretching if you are out and about. Use a heavy weight resealable polythene bag. Fill it with just enough water to do the job. Put the paper inside and seal the bag opening. Put it somewhere flat and just move it around to get the paper wet. Leave it to soak for a few minutes. Take it out of the bag and mount it. Use the remaining water for your painting.