I just finished making this. It was originally designed to hold a 12x16 glass palette in the bottom portion and all my painting supplies in the lid, but my measurements ended up being just a hair off. So I re-engineered it as you see it.
I used masonite for the top and bottom, screwed and glued to strips of oak. It now holds a 10x12 glass palette and I have 2 compartments for my supplies. The palette is held in place by a 1/4" square dowel on one side and a small removable pin on the opposite side. I drilled a small hole underneath so I can pop the palette out with a brush handle. For now I have some pieces of coroplast that i can wedge into the compartments to keep my supplies in place.
Here it is as I use it with my panel holder attached to the tripod.
The panel holder is made out of some scrap strips of wood I had and two 1/4" threaded rods. The bottom piece is secured to the rods with nuts on top and bottom and the top piece simply slides freely up and down. A panel is then basically sandwiched between the two pieces with some brass clips I found in my miscellaneous hardware collection. It will hold up to 11" vertically.
The panel holder fits inside the lid and is held in place by some pieces of square dowel. I would like to be able to use it to carry a wet painting, but when I close the box up and hold it by the handle the panel slides right out of the holder. I'll need to ponder that one some more, but for the time being I made a small wet panel holder out of coroplast that'll hold 3 panels, so that should be ok.
Here is the box closed up and the support that it sits on.
I have only done 1 plain air painting so far, but now that I have a more economical setup I will try some more. I just wish the weather here would cooperate!!