wherever you put the battery just remember you need to lift it in and out of the box so a cupboard is unlikely to work, as the box should be secured to the ground, i wouldn't install a battery unless it was enclosed to prevent spills, seen a couple that split, they destroyed the box but nothing else.
some other makers (Avan is one) put the battery in a sealed box vented to outside, which is the best method in my books, they can give off dangerous gases
not sure of your layout but i have seen one under the seat where a drawer once was, they just dismantled the draw and screwed the face back on from behind, it looked neat and tidy, but you lose a drawer .........