Another view of the battery securing system.
As you can see there are two red (positive) terminals and two black (negative) terminals on each battery. The outer ones are automotive-style post terminals -- although it's difficult to see, there is a small ledge of plastic surrounding them. This will help keep the angle-iron from shifting.
The inner terminals are simple threaded stud terminals, and come with a stainless nut of the appropriate size. We'll use these terminals for our wiring.