You are trying to seal a gap in a square piece of material that is affixed to the wall. Even though it is screwed into the wall. A gap still exist
You can caulk the top and the sides. But because of its position you can not see the bottom. There is about 7" space to the floor at the bottom
You would have to do by feeling with your finger tips. Maybe you could get the caulking gun under there. Too bad they don't have an attachment hose for a caulking gun for tight spaces?
Have a telescoping mirror to view the area
