Yay, you got the back off!

Next, you need to release the metal strap under the sticker that holds the battery. This strap is wrapped around a pair of plastic horns on either side.

To release the strap, you will need to insert a narrow and sharp object into the slot between the pair of horns, and pry the loop of the metal strap off of the plastic horns.

(NB: The picture with the knife was taken several years after the earlier one, the sticker has faded quite a bit!)