What batteries are those? The color makes me think lg he4, but the stickers on yours are different than mine. (My sticker is clear with black font, yours is white).
Got any other cells that you can pop in there to see if maybe its the batteries at fault?
Guessing here, cause to my eye your box mod looks pretty pro.
Aside from that, the only other thing I can think of off the top of my head is where your button and 510 connector pass through the body of the mod. Are you sure that there is sufficient metal to metal contact with the components and the box itself? If the box is painted or powder coated and the button and 510 connector can't make a good contact with the chassis on the ground side of things, I could potentially see things heating up more than would be desired. Take that with a grain of salt though, I guess, since I've never built one of these things.
