I'm going to suggest the Kingman Fasta 9v. It'll keep up with 12.5 just fine, and if you're good with a soldering iron and a dremel you can make it an 18v. It's $50, has nice capacity, and it's profile isn't really that big (Some people will say it's like playing with a whale, I never noticed an issue).
You may call it fat, I call it bounce padding.
I'm a huge B///MW fan