I used 2 U channel aluminum bars. The battery is 6s 18AH. its very slim and the cells fit into the U of the aluminum. The bars stiffen the deck so barley any flex at all and doubles as an enclosure for the cells since they will be the first things to get hit in an impact.
Ill be mounting all of my boards like this in the future. it gives easy access to the battery for repair and eliminates the need for an enclosure