I have always found that some L Shape parts that you can clamp in the vise are very useful. For this particular situation, you could clamp an L piece in your vise. Screw together a step piece from some scrap, and put it in front of the bench. Grab another piece of scrap to use as a brace for the far end. Like in the picture attached. If you box clever with the step piece dimensions, you can build just one, and rotate it when you want to switch from planing the long sides to planing the short sides. You can double stick some old rag, or cork or anything else to the l piece, brace and step to keep them from damaging your drawers. If you want to have some support in the middle of the drawer (not quite sure why you would if you are only working on the joints) take two pieces of wood, both of which would be smaller than the gap you want to support, and screw them together to match the gap you want to support.