May I suggest a battery box from a PT cruiser.
It bolts to a plate or tube ( with fabrication ), it does not need a top bracket, it is PLASTIC and will not rot and it has a great hold-down that makes putting the battery in easy ( delending on the location ).
I wish I had designed mine around this box/tray.
It is just a suggestion.
BTW, if you put in the dead center of the rear, wouldn't you have a problem getting the trunk open?
