You probably don't need the case, though you will want the space to be vented. There have been threads on here, where folks have taken the battery box off the front of their TT, and replaced it with a metal storage box, that you might have in the back of a Pick-up...and then install the batteries with brackets to keep them in place. This also allows for storage of some other things, that you might want to lock up.
They installed vent screens on the backside of the box, so that it wasn't really visible from the front.
As long as you secure the batteries and have the compartment you will be using vented, you should be fine.