A nifty device on eBay from orpheus_c, it uses the cage's handgrip as a large battery to power your Sony camera. Allowing for much longer run times than what you'd usually get from your FW-50 batteries alone. Curing one of the big complaints about this series of cameras, their poor battery length.
http://www.ebay.com/itm/Kovacam-186...-for-Kovacam-A7s-II-A7r-II-Cage-/121822159604
http://www.ebay.com/itm/Kovacam-186...-for-Kovacam-A7s-II-A7r-II-Cage-/121822159604