In stock at EB Airsoft right now is a Silicone Battery Case in the form of an M84 stun grenade. If you want to bring a powerbank that you can use in the field and will not make out of place then you can bring this. It can be used with two 16580 rechargeable batteries which are not included in the case.
eAiming Battery case available at Tiger111HK for US$36.33... "Compatible with Lithium Battery / NiCD/NiMH Mini type battery and with Polymer construction. This fits the standard 20mm rail so dress up your AEG with this beautiful battery case with stickers on the right position. Dimensions: Battery Compartment Dimension: L85mm (Extendable to 110mm) x W56mm x H22mm."
Another AN/PEQ16 battery case available at eBaybanned and just costs US$24.50... "Quality is 100% brand-new guaranteed. Can be configured as Battery Case or Dummy Laser Sight. Holds Standard 8.4V Mini Type Battery Pack with included Spacer. Remove Spacer for decorative dummy. Fits 20mm Rails. Reseller please call +86 13631396867 for wholesale. Free shipping."
The new AN/PEQ-15 Battery Case with Laser from Beta Project now available at Airsoft Buddy. We just wanna kid William regarding the name of the company since products will always be stuck in "beta" rather than final. Anyway, this was just launched yesterday so Airsoft Buddy is the first one who have this for sell. The selling price is US$22 and comes in black and tan colours. Airsoft Buddy highly recommends that your purchase this together with their WE SCAR-L GBB.