Ok, I have been too busy that I haven't been able to devote time to this project. Anyway, I picked up the attached box for $2 at a thrift shop. I was excited about the box because it is pretty sturdy and fits the battery with about 1/2 cm of clearance all around so I can pad it and protect the battery from banging around.
However, I have decided against it because 1) it won't allow me to keep the battery dry and 2) the box won't allow me to add some sort of door so I can access the battery if I have to. I want to have as much of a watertight enclosure as possible since I may end up riding in the rain sometimes.
I'm thinking now that I will add some sort of wood brace on the triangle to support the battery. then put the battery in some sort of pouch. I don't want to battery to just hang from the top bar since this can stress any kind of nylon even strong nylon.