Repurposed an old tool case from work. Little bit of foam cutting, and it's a perfect fit. The hard cases are nice, you can toss them in the back of the truck and not worry about it.
I bought all these boxes to help my organization in the garage. Once I got them home though I decided I'd be buying another one of the smaller boxes to store an SBR in. In particular my soon to be built 4" barrel 9mm AR. I don't know the measurements off hand, but I think it will just fit (disassembled). A warning, it would be a little on the small side for a lot of things. I still need to come up with foam, but the boxes are very well constructed, and I think would hold up to any abuse. Depending on where you frequent a tool box may not look too out of place at all. I want it for more of a sexy range bag and storage case than discreet carry, though it might fit your use okay.
The dimensions, internal, of the smallest ridgid box posted above is 20x11. The smallest SBR I have is 19" collapsed and with optics barely fit in the case. None of my other SBR's even fit in the case even after being taken apart. They are great cases, but have limited space