I posted a photo of mine with a Blackhawk double stack single mag case holding a Glock 31 round magazine. I got the idea here at KTOG. You can also find a YouTube video on how to modify the case to fit a standard rail.
The case has a center bar that needs to be removed, along with a small belt hook. The case is plastic, so I used a Dremil with grinding wheel to grind it smooth. It is a tight fit on the rail. You also have to remove the spring clip (small screws) and remove a small amount of the flange to fit down and over the rail. Once in place, the spring clip will hold the case from moving on the rail.
Remember, the case only holds the magazine in place by friction; whether that is good enough for your application is something you need to decide. My is a range toy; works GREAT!