If these are just for the serpent then maybe
http://www.hobbycity.com/hobbycity/s...idProduct=6421 would be nice. Some people don't like them because theyre cheaper and chinese made, but for around $100 for a 2500 6s pack you probably can't do much better. The seem to fit your size constraints stated earlier, it could mean making up a different mount though as you'd only need two. or do go a 3 pack setup they make them in 2s too, thin as well for lower cg.