When using a tripod for painting plein air with a pochade box, use the legs - not the head - to level your box. Start by really tightening all the head adjusting knobs, then set up the tripod and attach the box. Now adjust the legs for angle and length to level the box.
When installing a quick release plate, install it at an angle rather than square with the tripod mount. This will intrease the stability of the pochade box on the tripod and will reduce the tendency wobble.