I use 550 cord. I've got a few feet of it coiled up in my vest. I can tie it on for a temporary sling, carry birds with it, replace a broken boot lace, or use it for about a million other things.
It doesn't make the most comfortable sling, but it's OK for a while, especially if I pad the top of my shoulder with one of my gloves.
I believe that Steve (sm) prefers rawhide strips... but that's just 'cause he's old.
Somewhere in the basement, I've got a factory-made sling which attaches via a nylon loop around the stock wrist and another loop around the barrel. It works fine to carry a gun, but I don't like shooting with it on and it's a heck of a lot bigger and less useful than some extra 550 cord.
In fact, if the orginal poster would like my unused** factory sling, for free, I'll send it. Just PM me your mailing address.
** Corrected to add: The sling HAS been used, lightly. However, is not currently in use and is simply taking up space in a box of misc shooting junk in my basement.