Here are two views of my square pochade box, made from a cigar box I obtained from a local store for a couple of bucks. This one holds a 5x7 panel. I usually use 4 pieces of doubled over masking tape behind the board, to ensure that it stays put.
You can see how the little square pieces in the lid serve two purposes: to hold down the palette when the lid is closed and to support the panel you are painting on.
I cut four 1/2" pieces off the wood that came with the box for some reason (maybe someone who smokes cigars can tell us?) and carefully adhered them using carpenter's glue.
That same long piece of 1/2" square wood was glued at the back so the lid of the box will prop open, supported by the wood. I will post a photo of the back also. The square frame that inserts into the bottom of the box, and to which I have glued the paint sticks, serves also prevent the panel from falling into the paint piles on your palette when you shut the box. This square frame is very thin and fits outside the square pieces glued in the lid.
There is room under the palette for small tubes of paint and maybe a sawed off brush or two, although I usually carry another small bag with supplies like oil, a viewfinder and paper towe or ragsl.
The nut in the bottom was really easy to install, just drill the right size hole and hammer it in (there are diagonal claws that sink into the wood to hold it in place). I used a little glue as well to make sure it stays put. Take your tripod to the hardward store to get the right size.