It takes no more wire space to hook batteries in series than it does to parallel them. In fact it is actually easier. You can place each battery anywhere you want, and need only connect them together with one wire from the positive terminal on one battery to the negative terminal on the other. The other terminal on each battery is your positive and negative that is wired to your switch/distribution panel.
However, I do not know that I would trust a $10 battery box
- USCG approved or not. If I did use them, I would make sure they are securely tied down. The last thing you need is a couple of batteries bouncing around your cabin
I have two much larger 6v batteries on my boat (370ah, 113 lb. ea.) I built my own boxes from plywood
so I could securely attach them to a substantial structural member
. They cost more than $10 each, but I never have to worry about them coming out of the boxes even if I turtle the boat.
By the way, I lived in Charleston, SC (Mt. Pleasant, actually) from '84-93'. Nice place.