The Sling-arms are labled as an upgrade for Skavenslaves, but, really, they're for our Gutter Runners.
What I did was get a pack of those arms, and then look through the Clanrat box for the most Eshin-y looking models (there's one with a long hood, and one or two with masks).
I considered using the cloaks on the old Nightrunner spru, but
they didn't fit quite right, and look a little static on the new models.
The only thing left to deal with is that rigid left hand/arm. Simplest thing I did: take one of arms holding a katar-style weapons from the Clanrat spru; the one that completely covers the back of the hand. Cut off the weapon and grind down the hand holding it. Then, just glue it in place over that left hand.
Other options I've seen include sculpting a knife handle on top of that hand and gluing a blade on the bottom, or a net, or holding the edge of his cloak, like Eshin troops are surely want to do:
http://i42.photobucket.com/albums/e344/BobLipp/Nightrunner5.jpg