You know... I think I read in the modder's forum that someone just shaved it (the button, with a razor blade) down to just below the body of the battery, and it worked out well.
The newer PCC's already have a slot cut out for manual batteries. Mine does and I got it from E-Smokey Treats:
No reason to carry a spare battery because you can just trickle charge the one battery when you finish vaping and it won't overcharge the battery. You can also leave the cart on the atty that you store in it +2 spare carts. I haven't run the PCC battery dead once yet.