Vapemate seems to be the closest thing to a dedicated feeder mod that would work.
If you're mostly looking for capacity and don't mind cartomizers, you might consider a tank. The better ones run from $20 on up, but if you search around there's a relatively popular new low-end tank out there selling for well under that. In fact, there is at least one place selling the exact same thing as a "disposable" tank for just $5.00(!).
I have four of those tanks, two each from two sources. From the more expensive source, one is pretty much crap and the other is just OK. From the cheaper source, both are actually better than the other two. But I think that's just luck; quality control on these is a complete crap shoot.
If you manage to find these and decide to try them, I recommend getting the "normal" resistance (~3.0 ohms) for the Darwin and expanding the near-microscopic hole in the carto with a pick, nail, small punch, etc. They're all DC cartos and longer than normal but if you don't like them, get tired of them or don't like the longer form the tank body is not real thick and can be easily cut down to normal carto length. Personally, once you've got a good one that seals well I don't recommend taking them apart much -- just pull the old carto out and insert a new one when necessary.
If nothing else, they're a great introduction to tanks in practice and can help in making the decision on whether the expense of a higher-quality tank it is worth it to you. The only real problem with any tank on Darwin is that the swing arm cannot be closed with the tank attached. I think I saw a picture of one once in which the tank was actually rotated 90 degrees, but I don't see that as either attractive or practical.
Frankly I am surprised that no one has yet made a round tank for the Darwin with eccentric holes in the caps so the carto goes through very close to the tank wall (as opposed to dead-center as is the norm). If the caps were made just right and the body is not too thick, I'm thinking that would allow the tank to be rotated out so the swing arm could actually close. ...