- (void)viewDidLoad {
    [super viewDidLoad];
    self.mapView.delegate = self;
    self.mapView.showsUserLocation = YES;
}

// ...

// Later, I retrieve some models and generate CustomMapOverlay instances from them...
for (Model *model in models) {
    CustomMapOverlay *customMapOverlay = [[CustomMapOverlay alloc] initWithModel:model];
    [self.mapView addOverlay:customMapOverlay];
}

// ...

// Implement MKMapViewDelegate protocol
- (MKOverlayRenderer *)mapView:(MKMapView *)mapView rendererForOverlay:(id<MKOverlay>)overlay {
    MKPolygonRenderer *polygonRenderer = [[MKPolygonRenderer alloc] initWithOverlay:overlay];
    polygonRenderer.lineWidth = 2;
    polygonRenderer.strokeColor = [UIColor colorWithRed:0.0 green:0.5 blue:1.0 alpha:1.0];
    polygonRenderer.fillColor = [UIColor colorWithRed:0.0 green:0.5 blue:1.0 alpha:0.5];
    return polygonRenderer;
}

@implementation CustomMapOverlay

// Required by MKOverlay protocol
@synthesize coordinate,
    boundingMapRect;

- (instancetype)initWithModel:(Model *)model {
    coordinate = CLLocationCoordinate2DMake(model.latitude, model.longitude);
    double radiusInPoints = MKMapPointsPerMeterAtLatitude(model.latitude) * model.radius;
    boundingMapRect = MKMapRectMake(model.latitude, model.longitude, radiusInPoints, radiusInPoints);
    return self;
}

@end

CLLocationCoordinate2D centerCoordinate = CLLocationCoordinate2DMake(model.latitude, model.longitude);
MKCoordinateRegion region = MKCoordinateRegionMakeWithDistance(centerCoordinate, model.radius, model.radius);

int numCoords = 4;
CLLocationCoordinate2D *coords = malloc(sizeof(CLLocationCoordinate2D) * numCoords);
coords[0] = CLLocationCoordinate2DMake((region.center.longitude - 0.5*region.span.longitudeDelta), (region.center.latitude + 0.5*region.span.latitudeDelta));
coords[1] = CLLocationCoordinate2DMake((region.center.longitude + 0.5*region.span.longitudeDelta), (region.center.latitude + 0.5*region.span.latitudeDelta));
coords[2] = CLLocationCoordinate2DMake((region.center.longitude + 0.5*region.span.longitudeDelta), (region.center.latitude - 0.5*region.span.latitudeDelta));
coords[3] = CLLocationCoordinate2DMake((region.center.longitude - 0.5*region.span.longitudeDelta), (region.center.latitude - 0.5*region.span.latitudeDelta));
MKPolygon *polygon = [MKPolygon polygonWithCoordinates:coords count:numCoords];
free(coords);

[self.mapView addOverlay:polygon];

coords[0] = CLLocationCoordinate2DMake(
              (region.center.longitude - 0.5*region.span.longitudeDelta),
              (region.center.latitude + 0.5*region.span.latitudeDelta));

coords[0] = CLLocationCoordinate2DMake(
              (region.center.latitude + 0.5*region.span.latitudeDelta,
              (region.center.longitude - 0.5*region.span.longitudeDelta));