Do you use a tank? I can't give it up to be honest... I can't go back to dripping again...
I'm a CE2 fan and used a self-built tank some on my Darwin. Of course the arm doesn't close with the larger tank system attached, but I had no problem leaving the arm up at about 160 degrees or more and tucking Darwin on my back pocket with the screen facing outward and the USB end down in the pocket. Drew offers a couple of different sizes of belt holders that work great with a Darwin.
I don't have one of the nice new belt sheathes with mine (not really needed for me), but with one of those you can probably strap it right on and walk around with a large tank no fuss. I do think new Darwin orders come with one
Here lately I've ditched the tank and been enjoying the eGo sized stuff for long days on the road...many of those can hold dern near 4ml of liquid and will usually last all day for me no problem. Several types and brands out there these days, things like Giantomizers, Visions, Echo, Mega510, Ego Sized Dual Coil, and the list just keeps growing.
The eGo sized cartos all open and close for me on the Darwin since I hook them up with a few extension pieces.