While watching a video by Free(k)hand showing one of his watercolor sketchbooks I spied a spread about his watercolor box holder and decided to make my own. While not as tidy as his, mine works great for my purposes. The box is an Altoids gum box, spray painted white on the inside with Rustoleum. I used double-sided carpet tape to adhere the half pans, filled with my favorite colors, to the box.
I located a metal bookmark and glued velcro to the head of the bookmark using Tough As Nails glue, and the companion side of the velcro is glued to the bottom of my paint box.
To use this little contraption when I'm out painting I simply take the bookmark out of the pocket in the back of my Moleskine and slip it onto a page of my notebook with the head sticking out at the top and fasten the paintbox to the bookmark and paint away, using a waterbrush. How cool is that!