I'm afraid the situation is NOT as you have assumed. The fact is, I have THREE 12v12a/h batteries sitting around, and I don't really have enough money to buy Lithium Batteries.
I have made a mock-up of the battery box, but now I think I will mount it Inside the frame, using the bottle cage screws . In fact, it might be more like a cage to hold the Battery, as there is an issue with inserting and removing the Battery once the box is attached to the bike.
I wanted to keep the Center of Gravity low, but the box would be so low, I'm afraid it would strike, or get hung up on, the curb, if I was to go over a curb. And while test fitting, I realized the battery would get blocked by the Kickstand, unless I make the fiberglass keel longer by about two inches, which would make the box even lower.