What's wrong with a short piece of bungee cord?

Batteries are heavy, hard objects that I would prefer not hit me in
the back of my head in heavy turbulence or worse, a crash landing. I
would never trust a bungee cord to restrain a few pounds of lead! I
use a 1" nylon strap (heavy duty) with a METAL BUCKLE (I have broken
too many plastic buckles to trust them in a critical application)
attached to some solid structure of the glider. Buckle is positioned
to keep it far away from the terminals (which are insulated too)

In my glider, the (2) 12 Ah batteries are sitting in a aluminum angle
(1.5") tray bolted to the structure. This way the tray absorbs most
of the g forces and the straps keep the batteries in the tray. By the
time the straps break or the tray tears loose, I probably no longer
have to worry about the batteries hitting my head.

I also use PowerPole connectors with fuses on each battery.
Connectors are double sided taped to the battery as a strain relief.

I also carry a auto cigarette adapter with a powerpole connector and
my cell charger so I can charge my cell if I get stuck somewhere.