You said you normally carry your cigarettes in a case, right ?
The case should be long enough to carry a penstyle battery if there is a couple of milimeters of spare space at the end of a normal cigarette.
You may also be able to fit the atomizer in the case as well and perhaps some extra carts. Although the atomizer and carts maybe to fat to allow it to close.
Cigarette cases are not expensive and should be easy to buy in Singapore, you might want to look around and see if you can find one there to fit your penstyle when it arrives. I have seen double layer cigarette cases for sale, that's cigarettes stored in both sides of the case.
To keep them safe try putting a cleaning cloth used for eye glasses in there as well, they are only small but are soft and should protect things from damage.