Mounting the Battery
The battery is a relatively heavy object, and there is much more weight in the rear, than in the front. In order to balance the car, many builders elect to install the battery in the front, commonly in the passenger footwell, or in the radiator compartment. Moving the battery to the opposite end of the car from the engine is standard race car practice.
The tradeoff is that the wiring to put the battery in the front adds weight that is often not considered. Also, in order to maintain voltage from the battery to the loads, extremely large cables must be used when the battery is located far from the powercells. Finally, such large cables require very large tools to crimp them properly, and represent a cost to acquire if needed.
When a modern relatively lightweight battery like the Oddysey PC1200 is used, mounting it in the rear (where the harness expects it) may be so light that mounting it in the rear may not be a net weight savings.
With really light Lithium batteries, mounting in the rear is likely to be a net weight loss, which is the goal, and the effort and weight to move it to the front would not be worth it.
This illustrates the perhaps unexpected conclusion that the lighter the battery, the less advantageous it is to spend the weight to move it to the front- with a risk that moving it forward may actually increase the overall weight.
As a result of this and other factors (it's easier to mount it in the back, voltage drop is much less of a problem, etc.), many builders choose to mount the battery in the rear of the car, usually on the driver side. For balance, the passenger side is probably the better choice, however.
The last best reason to mount the battery in the rear is that the InfinityBox harness expects it to be there. That makes it the easiest place to install.
In any case, you will need to buy or fabricate a battery box to secure the battery.