I saw a photo of Joseph Zbukvic doing some plein air urban sketching using the Winsor & Newton fieldbox. It looked pretty practical with integrated water container, so I just ordered one from amazon.de with the Cotman watercolors, a lot cheaper there than in the Netherlands.
For pics and other portable palettes read this nice review by Daniel Novotny: https://www.ateliernovotny.com/2017/07/21/101-4-6-how-to-set-up-your-watercolor-palette-portable-palettes-paintboxes/.