The bottom fiber board is served as a protection to the battery against crash. Mounting the battery on the top with a rubber band, there will be a shock absorption buffer during crash. Pilots may also add an anti-slip sheet between the battery and the frame to keep the CG.

This setup will give you a bigger protection to the battery. When you have a hard landing, the ski hit the ground. Then the battery has the inertia going download. The rubber band will let the battery move but absorb some energy. The lower board (or extended fiber board) will protect the battery from hitting the ground directly.