I keep my Mossy 500 behind the bed - I have a tall headboard, and the gun is inaccessible my 4 year old. I keep shells in the side saddle, but an empty chamber with slide lock and safety on.
Also, curtains cover the shotgun. All I have to do (or my wife) is reach behind the bed and pull the gun out.
I used hooks and drywall anchors to secure the gun. The hooks are about 6" out from the wall, so the gun lays flat/at a slight angle, and about 48" off the ground. The bed is up against the hooks, so the gun cannot be removed unless the bed is moved (heavy king bed, on carpet = difficult for an adult to move). My son cannot reach behind the bed and retrieve the gun either. I measured his arm and checked the distance to the both edges of the gun.
If you are going to have a HD shotgun, I don't see the point in locking it up.