Mugen is not ready yet with their battery jacket for the HTC 8X and Nokia Lumia 920, but they do have an extended battery available for the Nokia Lumia 820 in stock now.
The 1800 mAh battery is 9% larger than the stock battery and fits under she standard battery cover of the device, so does not add any extra bulk.
It is however pretty expensive for what it is, costing $46.95, but if you are planning to buy an extra battery to get you through the day you might as well get one that will last a little longer.
See the battery at Mugen here.
via Windowsteca.net