My friend, Fred, was able to translate my doodling into something that would use the existing hole pattern.
| Left: Holder I had made, Right: Included holder designed for 12v 7ah batteries | 
Installation is pretty straight forward...
But I noticed we had a small issue getting the the batteries into the outer sides.
| The round battery frame rods extend into the battery holder space..... | 
I was pondering a few ways to trim the round aluminum rods and thought i would ask Fred for his thoughts. Hacksaw? Dremel? File? No! Bring it over, let the CNC do it!
We used a file to remove whatever remained.
And here's how it all looks now...
Now 41 pounds of batteries no longer required an awkward bungee cord and bracket to stay in place!