How about a pack of these Brian:
http://www.cheapbatterypacks.com/mai...an=ELT&cat=E30
If you scroll down, there's an Elite 3300mah 3S 30C pack. It's within your space constraints, has plenty of discharge capability, and a good price too. Only thing is that capacity is a tad lower than what you say you want, but just thought I'd throw the idea in there. They're meant to be excellent cells from what I've heard.
