This Ryobi Double Battery Mount holds two batteries at a 25 degree angle.(The photo shown is 5 of these beside eachother) Benefits to this battery mount over other battery mounts includes:
High storage density for wall storage / efficient use of space. Many other battery mounts require space above to lift the battery out of the mount, this does not.
Battery is oriented for quickly checking battery level with a press of the button.
Slides in and out easily without requiring a press of the battery's side spring clips.
 
 
Compatible batteries I have personally tested and confirmed with a good fit:
4Ah  works with the following 18V One+ batteries:
4Ah Standard battery: PBP005
4Ah HP battery: PBP004
6Ah HP battery: PBP007
 
2Ah works with the following 18V One+ batteries:
2Ah HP battery: PBP003
It is possible it works with others, but these are the only ones I have and was able to test.