I've been using the Sinclair Montana Sling from Brownells for around the past 10 years. Very fast to adjust the length/tension for shooting positions or adjusting to fit clothing. While it can still slip off your shoulder, you just have to hold on if that's how you carry it. I mainly use the sling for shooting positions. Another good sling if you're concerned with carrying it and slipping off your shoulder, is the "Claw" sling with rubber on it.

Montana Sling:
https://www.brownells.com/shooting-a...prod22874.aspx

Claw Sling:
https://www.brownells.com/shooting-a...-prod9933.aspx