I think that the best way to secure the battery is by building a box that will be screwed to the structure of the plane.
I make a test box with a 0,020 steel sheet. The result looks good but a bit flimsy, I will try to find a thicker sheet.



Here the box is in the fuselage. Final location is to be decided as it has to be accessible for maintenance.



And another location, in another bay under the seats.