I had plently of front/back support so it will not move around much. But I also wanted to prevent any lateral/side-side movement. I really didn't want to add to much more metal to an already heavy droid. So, I went for simple and practical....bungee cord!
I measured the distance as 23 inches to go from the plate, over the batteries and to the other side. At the hardware store, I found the industrial strength bungee section and found a 20 inch one that would work perfect.
With the large hooks, the bungee winds up being a loose fit since it measures 25 inches long. Simply tying a knot in the bungee cord brought it down to 22 inches, making for a very snug fit!