I have never done this, but I was told that it worked pretty well.
Paint a 2 liter bottle black, fill it with water, lay it on its side.
Poke a small hole in the cap.
When the sun heats up the water it will pressureize and start to come out the hole in the cap.
I think only half of the vessel is usable, after you get below the hole on the cap you loose the water pressure.
It would seem a vessel more like a cylinder may be a better choice, it would empty the vessel almost completely.
But 2 liter bottles are easy to come by.
Also instead of just black, take a few shades of paint like green grey and brown.
Spray or dab some on a small branch with leaves still on it and just tap the vessel with the paint covered branch to give a very natural camo look.
Don’t get too crazy, go very light on the colors.
You only need a little color to break up the solid black, otherwise the eye will pick up on the shape and outline, just a few lines here and there to mess with the eyes and brain.