I have an Obsidian 9 myself, and did a lot of research on it. I settled on a Strike Industries Strike rail. It has an I.D. of 1.5" and can accommodate a can tucked inside. I have a 4" barrel with a 7" rail. Make sure you get the tri-lug adaptor for both the barrel and can. I use the Rugged adaptor for the can and a Hansohn Brothers for the barrel.
Other rail choices: Samson Evo, Midwest Industries, Seekins MCSR, Griffen Armament, and the DD MFR XL.
Edit: inleadwetrust also makes a tri lug flash can that will fit under a rail for when not using the suppressor.
Here is my AR-V with the SI rail and Obsidian 9: