I went with the Flightpower 4s 4270mah pack that linc recommended and it is a perfect fit.
I can hopefully make the battery holder/tray this weekend and maybe even give it its first test run on the MM ESC from my Revo.
http://i199.photobucket.com/albums/aa3/jnev101/CRT5%20Conversion/DSC02130.jpg
http://i199.photobucket.com/albums/a...n/DSC02132.jpg
http://i199.photobucket.com/albums/a...n/DSC02133.jpg