Someone might want to try MindShift Gear's GP 4 battery pouch. I don't have an NP-W126 battery to test the fit, but it works great for both my Fuji NP-95 and Sony NP-FW50 batteries. It's similar to the ThinkTank battery pouches (they are sister companies), but designed for slightly smaller batteries. It also has zippered compartments for SD cards and a business card window on the other side. Not only that, but it's slightly cheaper than ThinkTank battery pouch, and much cheaper than buying 2 of the LensCoat pouches mentioned earlier. To keep track, I put contacts up for fresh batteries and down for spent ones. Couldn't be more perfect for what I use it for!
With NP-95 batteries:
With Sony NP-FW50 batteries:
Business card slot: