My husband fixed up a box for me when I went to England last year to paint with friends. I didn't want to carry my French Easel, too heavy. So he changed an old watercolor box I had, into a version of the Heilman boxes, which are really nice, but very expensivehttp://www.heilmandesigns.com/. I also googled "how to make a plein air painting box" and found some good ideas. Judson's have some really good boxes, or they could give you ideas on what you might like if you wanted to make your own, http://www.judsonspleinair.com/ Those are both US companies. You can also send in information about yourself and your plein air painting, and Judson's will many times use you as one of their "featured artists". Good luck!
I can loan you one tomorrow and strongly recommend the Julien box easel or half box.
I'll bring a half box for you tomorrow. Assuming you're going to attend!! John