Background mesh is basic (initial) mesh to be refined. All the surface model must be placed inside the background mesh. It is created using blockMesh utility. The background mesh is simple block a bit larger than bounding box of the surface model. Alternatively, the background mesh can be used to create inlet or outlet of the final mesh (this case).

Example of blockMeshDict file:

convertToMeters 1;

x1      -75;
x2      105;
y1      -77;
y2      85;
z1      330;
z2      450;

vertices
(
    ($x2  $y1  $z1)
    ($x2  $y2  $z1)
    ($x1  $y2  $z1)
    ($x1  $y1  $z1)
    ($x2  $y1  $z2)
    ($x2  $y2  $z2)
    ($x1  $y2  $z2)
    ($x1  $y1  $z2)
);

blocks
(
    hex (0 1 2 3 4 5 6 7) (12 12 6) simpleGrading (1 1 1)
);

edges
(
);

boundary
(
    w1
    {
        type patch;
        faces
        (
            (1 2 6 5)
        );
    }

   w2
    {
        type patch;
        faces
        (
            (5 6 7 4)
        );
    }

   w3
    {
        type patch;
        faces
        (
            (4 7 3 0)
        );
    }

   w4
    {
        type patch;
        faces
        (
            (0 3 2 1)
        );
    }

   w5
    {
        type patch;
        faces
        (
            (0 1 5 4)
        );
    }

   w6
    {
        type patch;
        faces
        (
            (2 3 7 6)
        );
    }

);

Run blockMesh:
 blockMesh