More and more frequently I am using a Sienna All in One Pochade Box for my outdoor painting. It is especially nice when I stand up to paint, which is what I usually do outdoors. It is compact, built well, and has lots of features not found in other boxes. It is easily adaptable to both oils and watercolors and rests comfortably at any height on a sturdy tripod.
The Sienna will comfortably and securely hold a 16 x 20 panel, which is the largest I use outdoors. My watercolor paper is taped to a sheet of Incredible Art Board which is extremely light and easy to carry. I make a matboard flap which covers the artwork. Smaller pieces of 6 x 9 paper and another sheet of art board stores neatly underneath. Brushes, paints, pencils, and acordian water cup are stored inside.
The Sienna Pochade Box in use for a demonstration at Zion National Park.You can see how I lay out my palette and supplies to work outdoors.