Setting up a travel kit complete with everything a painter might need to paint outdoors takes a bit of resourcefulness. Many companies make travel kits with varied designs, all including paint wells for six to twelve colours and a mixing tray in the lid.
Someone, a bit of a design genius, came up with a brilliant design for a travel box based on the Altoids box. (Actually there are any number of brilliant things an Altoids box can be. Look up the Altoids box camp stove for one). There are plans for the watercolour kit on line. I made one.
Mine has two more colours and fits two brushes and a pencil (and probably their brush as well). Theirs has some wasted space under the plastic liner and also because of the window in the lid rather than a mixing space. The colours are quite nice though. I did a little sketch, in the sketchbook and not on watercolour paper. And yes, I did use that little tiny brush.