If you can't make your own, then you can fit the Pirhana Battery tray
http://outdoorauto4x4.com/p/4584109/suzuki-jimny-dual-battery-tray.htmlIf you get the tray above, then the biggest(best) battery that fits (without spending stupid amounts of money) is this one is an Optima Yellow top D51 (or D51R if you prefer). It's only 41AH, but all the other specs for this battery are superb for a battery of this size. And it doesn't mind being run flat like a deep cycle, but can also be used as a starter or even to run a winch cause it also doesn't mind high current.
I had to modify the tie down bracket to hold this battery on the pirhana tray.
You can see why people make their own to fit a larger battery, because 41AH ain't that great.
You'll still have room for ARB compresspr behind passenger side headlamp.