Domestic Battery bank

At present I have 2 battery banks 1 main battery and 2 on the domestic side The problem I have found is that the space available under the bunks in the stern cabin is of such dimentions that large capacity batteries do not seem to fit width 350mm and possibly the only option is to put 3 standard size batteries.

Any one have any experence of this, know of suitable batteries or modified the locker etc