[Editor: Much discussion on this topic. Here are a few pictures of such installations…]
Batteries sit in a fiberglass box. Box usually has walls only and when fitted in place, they build bottom inside of it.
I had to cut part of the floor above the box, between stern wall of the box and stairs. Floor can be cut and battery box can be extended to fit four 6V batteries. There is no need to change anything on floor boards. When you remove battery box lids with the attached floor boards, you will notice that floor under is not flat.
Here is a picture I found on internet. It is very much like mine. I can put in four 6V batteries from boat centerline toward starboard side. There is still space left on the starboard side for one more battery.
I’ve attached a picture of the 4 Trojan