You have several choices. Obviously not in the engine bay, unless you remove the air filter box and put a K&N pod filter on the front of the turbocharger, saving TONS of space! Actually, do NOT do that, the air box helps remove moisture from the incoming air stream and the pipe heading out of it contains the MAFS which measures the amount of air moving through.
So, your choices are in or under the tub. In obviously takes up space but if you build a box over a wheel arch you'll minimise the room taken (this is what I've done). I've also seen people put the battery and charging gear up against the front wall of the tub, effectively shortening the tub by a small amount, but it looked really neat.
Under the tub means you have to use a sturdy cradle that can withstand the pounding of 40kg of lead when moving (and if your roads are not in the best shape, factor in a little extra protection). You would also need to seal the battery terminals once you've done them up (they sell a sealer for this purpose, just spray it on and let it set).