My 21 lb is the B3121 at 550 CCA, it is too big for your purposes at 5.2" wide (but still a TINY car battery, easily fits in the backbone tunnel of my GTM). The 17 lb is the B2317 with a 4" width, 475 CCA.
The B2618 shows a 3.5" width, 18.5 lb, 472 CCA.