To make a battery flap, you'd need a few millimetres if you wanted to make it robust enough.

And honestly, since the battery is going to last for many years, there's no point in going to all that trouble. It's easier just to dismantle the Karma and solder in a new battery. It's not that tough to take apart and put back together as long as you know what you're doing.

The tough part is sourcing the replacement battery.