If you can't find one, and you can get your hands on a nice bit of old webbing around 2 metres long, you could probably knock something out in less than a half an hour. Particularly if you're lucky enough to find a bit of webbing with a loop already in one end.
I used this one when I had to carry my 10.6 munoz long distances, it was not worth steeling so no one ever lifted if from the beach while I was out.
About the only other things you'll need to make one are:
A way to make the holes and seal the webbing at the same time- I use a soldering iron
Some way to secure another loop at the other end if you want to make the sling adjustable - I used and old Karabiner
It takes around a half a nano second to get the point of balance right, and even if you don't get it perfect, one had on the broad is enough to keep it balanced.