Pulling power from the battery for an accessory is a doddle and you don't need to move the battery!
Slightly to the front of the battery is a battery jumper post. This unscrews to allow a connector to be slipped under it to draw power. You can connect a negative anywhere on the metal frame of the bike, there are loads of possibilities for that.
Be aware that you must put a fuse in as close to the connection as you can.
Simples!