I usually use 1x6 deck wood (stronger than fir, for example). I lash them to the stanchions. A pair of boards, upper and lower, is more secure than a single
board, but I've done it both ways.
On one particularly rough trip I had used lighter 1x4 boards, and they were broken by the bashing they took as we were pounding into the heavy seas. This may have been a good thing, since it may have kept the stanchions from bending or breaking. It's actually not particularly ship-shape to attach jugs to the stanchions, since this can subject them to brutal loads. Still, many of us do this, and we usually get away with it.