Did you take a look at Magpul MS-2 Sling? One of the best sling I've seen so far.. Probably in my next airsoft order. You can get it pretty cheap here : http://www.airsoftpark.com/replica-m...od-p-1616.html
. There's few reviews lying around airsoft forums about this replica and so far, no negative review. For 20$ you can't go wrong.