I managed to modify & fit 4- 6 volt deep cycle interstate batteries under the steps of my ws31 (same as the 31w) The battery tray is big enough the fit four, it's the height that is too much. They are 11" tall compared to some agm's at 9.65". Try to find a 6 volt AGM, they are closer to a 12 v in height.
INTERSTATE SITE-
https://www.interstatebatteries.com/...ies/deep-cycle
https://www.apexbattery.com/apex-apx...m-battery.html