We have the dual 6V batteries in our front storage box. The factory supplied plywood cover is perfectly adequate, but I occasionally knocked the retaining rods out of the slots with all the clutter I keep in there. I made up a new one of thin birch plywood, varnished, and drilled two holes instead of the slots. This way it takes a few seconds more to remove the cover, but the hold-down screws stay in place.
Last summer I did something similar retaining the original plywood cover and adding a 2" plywood strap with holes on top to retain the bolts in place. I also cut down the length of the bolts so the tops would not snag things as I passed then under the aluminum bars above the batteries. Made a big difference.