If its sealed with a clearcoat then you'd have to cut very carefully around the seams with an exacto knife to pull it apart, then replace the battery and add another clearcoat over it.
If it were me, after going through all of that Id find some way to attach it with rare earth magnets and not worry about it being watertight.
You guys might try over in the mod section for someone who works with wood.
That was my thought.....18 or so layers of cleacoat make for a bit of a challenge unless one knows what their kinda doing but not that a big a deal if somewhat knowledgeable working with wood.