A THUMB BOX!!
Definition Thumb Box: "A small box that contains painting materials, a panel for mixing colors, and a small canvas or boards and has a thumb hole or other device by wich it can be held upon the thumb like a palette to make a small sketch usually in oil." - Merriam-Webster
If you want to make one for yourself, don't worry about plans and instructions, just get a box and design it around your needs. The golden design rule "form follows function" is spot on!
I store four tubes of Gamblin's FastMatte oil paint (white, ultramarine blue, cad red light and cad yellow), two brushes with the handles cut down so they fit in the box, a palette knife and a little bit of Gamblin's solvent free gel in the box and keep the kit under the seat in my truck, along with a 5"x7" primed panel. If I have a few minutes and an intriguing scene, I just grab the kit and paint!