Second battery - new air filter

I've got a 96 XJ 4.0 and want to put a second battery under the hood. I do a lot of back country camping and I'm tired of packing a second battery and cables in the cargo. I know there are off the shelf kits (too much $), but I also know that replacing the air box with, say a K&N filter, you create some extra space that can accommodate a battery tray. So my question is: can someone steer me to the correct (best) replacement filter for my application, that is the set up that will leave me with the most extra space. I'm lazy, so just telling me a K&N won't help, I was hoping for specific information on which filter and associated 'plumbing'. By the way I'm in California, so I'll have to have a way to keep all those stock hoses hooked up. Thanks, Farmer Dean.

The Turbo City intake goes over the valve cover, leaving room where the factory airbox was for a second battery. It's CARB legal so it should pass visual inspection.

http://www.turbocity.com/product_info.php?cPath=17_25&products_id=104

Try this link, I hope this helps.

http://www.madxj.com/MADXJ/technical/technicalfiles/ARbattRelocation/BattRelocation.htm

I used the Spectre filter and air box;


I have a write up on here;http://www.naxja.org/forum/showthread.php?t=1026480&highlight=spectre

I have the ARB snorkel but it could be adapted many different ways, even to the pass side if needed. I have room for a second battery now.

Thank you for your help and suggestions, these were the type of responses I was hoping for. FD.