The ESP Guitar Company

If you can find the space, they make insulated battery holders (electrical & water resistant) that surround the battery and keep it from shorting out anything and also hold it in place.  EMG has them, when you purchase one of their 9volt Procell batteries at 5.00$, you get a insulated protector,  just goto www.emgpickups.com and check them out under the parts tab, you won’t see the insulated protectors, they come with the battery.  Maybe you can lay the insulated protected battery on top of an electrical component without fear of shorting.  Or maybe you can take a router and extend your electronics cavity a bit, but you would also have to make a new cover for that.  Just a thought.

EMG sells an external power supply. No batteries inside the guitar needed.