public BoundingBox BuildBoundingBox()
    {
        // Create initial variables to hold min and max xyz values for the mesh
        Vector3 meshMax = new Vector3(float.MinValue);
        Vector3 meshMin = new Vector3(float.MaxValue);
        foreach (ModelMesh mesh in model.Meshes)
        {
            foreach (ModelMeshPart part in mesh.MeshParts)
            {
                // The stride is how big, in bytes, one vertex is in the vertex buffer
                // We have to use this as we do not know the make up of the vertex
                int stride = part.VertexBuffer.VertexDeclaration.VertexStride;

                VertexPositionNormalTexture[] vertexData = new VertexPositionNormalTexture[part.NumVertices];
                part.VertexBuffer.GetData(part.VertexOffset * stride, vertexData, 0, part.NumVertices, stride);

                // Find minimum and maximum xyz values for this mesh part
                Vector3 vertPosition = new Vector3();

                for (int i = 0; i < vertexData.Length; i++)
                {
                    vertPosition = vertexData[i].Position;

                    // update our values from this vertex
                    meshMin = Vector3.Min(meshMin, vertPosition);
                    meshMax = Vector3.Max(meshMax, vertPosition);
                }
            }

            // transform by mesh bone matrix
            meshMin = Vector3.Transform(meshMin, meshTransform);
            meshMax = Vector3.Transform(meshMax, meshTransform);
        }
        // Create the bounding box
        BoundingBox box1 = new BoundingBox(meshMin, meshMax);
        return box1;

    }

class MyGame{
 BoundingBox theBox = null;

 public MyGame(){
   theBox = BuildBoundingBox();
 }

 public bool Update(){
   //Do stuff with theBox here:
   theBox.intersects(someOtherBox);
 }
}

public class Entity
{
    private Vector3 boundsSize;
    private Vector3 boundsOrigin;
    private Vector3 position;
    private BoundingBox bounds;

    public Vector3 Position
    {
        get
        {
            return this.position;
        }
    }
    public BoundingBox Bounds
    {
        get
        {
            return this.bounds;
        }
    }

    public Entity(Vector3 pBoundsSize, Vector3 pBoundsOrigin)
    {
        this.boundsSize = pBoundsSize;
        this.boundsOrigin = pBoundsOrigin;
        this.SetPosition(new Vector(0, 0, 0));
    }

    public void SetPosition(Vector3 pPosition)
    {
        this.position = pPosition;
        this.bounds = New BoundingBox(
            new Vector3(
                this.potition.X - this.boundsOrigin.X,
                this.potition.Y - this.boundsOrigin.Y,
                this.potition.Z - this.boundsOrigin.Z),
            new Vector3(
                this.potition.X - this.boundsOrigin.X + this.boundsSize.X,
                this.potition.Y - this.boundsOrigin.Y + this.boundsSize.Y,
                this.potition.Z - this.boundsOrigin.Z + this.boundsSize.Z),
    }
}