Securing your AR/AK; Outer diameter of standard handguards?

[IRO-bot]

So, I have a small safe. I'm not ashamed of it's size because it's perfect for my home. It is full of hunting guns, though, and my AR is hard pressed to fit. Literally. I have plans to build a wall mount in my bedroom closet using the lock from a police patrol car. I'm toying with the idea of hooking the 12v release to a keypad and all that. Total cost of hardware is right around 100 bucks.  The lock is an old style meant to fit on standard carbine hand guards, and I want to be sure my ALG tube will fit. It's super slim, but if anyone has a tape and a rifle handy, I would appreciate some measurements. Particularly outer diameter of both ends as well as circumference of the skinny ones. I have a set of the fat ones, but I'm not sure if the lock was meant to fit them. Better safe than sorry. For anyone who wants to build a similar set up, i'll update with finished pictures. The locks are about $40 on ebay, and you can find butt plates and mounts made by Santa  Cruz if you want less fabrication. Mine will be built in a triangle box, so it holds the rifle at an angle muzzle up and clears any sight I put n the gun. Since i store my rifle "Condition 3," I want to incorporate a support to hold the weapon at the end of the receiver, so the locked rifle can not be charged. Thoughts and ideas are welcome.