Zerotech Dobby battery case

I've bought a used Dobby drone without a battery lately and because of the not so cheap original batteries I decided to design a battery case for LiPo cells. In this case I've used two 1200mAh Turnigy nano tech cells, which have a round shape.

Update: unfortunately these cells have a massive voltage drop under heavy load. This causes the Dobby to land after 6:30 minutes although there's still a third of the capacity left in the cells. The cells are getting quite hot. The 15C rating doesn't re