That would be under the rear drivers side seat on a max wouldn't it? On my 14 xrs 2 seater with the second cvt intake it really only runs thru the back corner of that area. I'm in the process of making a tray to fit my second battery there.
There have been quite a few previous discussions on this subject. You might look into those. Also, you might consider replacing the OEM battery with higher capacity battery in the same location. Worked great for me.