But a Group 31 is a 12 volt battery and usually a Marine/Deep cycle, not a true deep cycle. He currently has six volt pairs (2 pair). I found no six volt DEEP cycle under 11 inches, the only ones under 11 inches were marine/deep cycle (US-battery) they also had one under 10 inches but it was a small (Capacity wise) Battery as well.
I would visit the usual suspects web pages
Deka (East pen battery)
US-Gear
Interstate
Trojan
And if you like AGM batteries LIFELINE.
They all have charts which list battery sizes and then find a dealer and if needed have him order what you need.