I can't quite tell from your picture exactly because the heat shrink Panasonic uses is sometimes grey, but is there any heat shrink on those batteries at all? Especially with lithium batteries, you should always have heat shrink on the battery or else shorting is only a process of a few millimeters (from the entire case [anode] to the button/flat top [cathode]) as opposed to from the bottom all the way to the top (such as a stuck firing button).
I just image searched 'Panasonic IMR' and came up with this image
If these are the batteries you are using I suggest you put an additional layer of heat shrink around the battery and ensure a few millimeters of heat shrink form a margin around the outside circumference of the bottom. I'm surprised they even come this way, as having that much exposed case could lead to some pretty nasty shorts from simple absentmindedness (inserting the battery backwards for example)