The Firearm Blog mentions the new holster High Threat Concealment is making. HTC has been quietly building a reputation and they had an interesting holster at the SHOT Show. They politely declined my request to take a photo of the new holster, (it is still in development stage) but the description on TFB should give some ideas.
The holster was quite effective as the AR was not visible until the person pulled it out. It had a 30 round magazine, and a 20 round magazine would make it even more concealable.
From: The Firearm Blog