Look no further than Safariland. They have a few different levels of retention holsters that will work with Glock 21s. For open carry it's a very, very bad idea to buy a holster that doesn't have some sort of retention system.
The Safariland 6377 is a pretty solid belt holster (actually the paddle version is, I believe, the one that L-2 pictured above). I use one for my G17. It runs about $50 and it has an active retention system that requires you to depress a lever to de-holster your pistol.
If you want to spend more money you could get a lot more hardcore and buy a Safariland 6360. The 6360 is more heavy-duty and is marketed more towards LEOs than civilian buyers. It has an even more advanced retention system with two different mechanisms in it. It's a very good holster, and it's gonna run you around $100.