Attaching 2d objects to 3d obejcts in OpenGL

Question

I am trying to render a 2d object at the side of a 3d cube that I have made. The cube rotates, and in doing so it will also rotate the 2d object with it. How to I attach the 2d object to the side of the cube?

I tried many ways including making the 2d object into a bitmap and then attaching the bitmap to the cube as a texture. But I felt like this isn't a professional way to do this.

3D Cube

void DrawCube(void)
{
     glMatrixMode(GL_MODELVIEW);
    // clear the drawing buffer.
    glClear(GL_COLOR_BUFFER_BIT);
   glLoadIdentity();
        glTranslatef(0.0,0.0,-10.5);
    glRotatef(xRotated,1.0,0.0,0.0);
    // rotation about Y axis
    glRotatef(yRotated,0.0,1.0,0.0);
    // rotation about Z axis
    glRotatef(zRotated,0.0,0.0,1.0);
  glBegin(GL_QUADS);        // Draw The Cube Using quads
    glColor3f(0.0f,1.0f,0.0f);    // Color Blue
    glVertex3f( 1.0f, 1.0f,-1.0f);    // Top Right Of The Quad (Top)
    glVertex3f(-1.0f, 1.0f,-1.0f);    // Top Left Of The Quad (Top)
    glVertex3f(-1.0f, 1.0f, 1.0f);    // Bottom Left Of The Quad (Top)
    glVertex3f( 1.0f, 1.0f, 1.0f);    // Bottom Right Of The Quad (Top)
    glColor3f(1.0f,0.5f,0.0f);    // Color Orange
    glVertex3f( 1.0f,-1.0f, 1.0f);    // Top Right Of The Quad (Bottom)
    glVertex3f(-1.0f,-1.0f, 1.0f);    // Top Left Of The Quad (Bottom)
    glVertex3f(-1.0f,-1.0f,-1.0f);    // Bottom Left Of The Quad (Bottom)
    glVertex3f( 1.0f,-1.0f,-1.0f);    // Bottom Right Of The Quad (Bottom)
    glColor3f(1.0f,0.0f,0.0f);    // Color Red
    glVertex3f( 1.0f, 1.0f, 1.0f);    // Top Right Of The Quad (Front)
    glVertex3f(-1.0f, 1.0f, 1.0f);    // Top Left Of The Quad (Front)
    glVertex3f(-1.0f,-1.0f, 1.0f);    // Bottom Left Of The Quad (Front)
    glVertex3f( 1.0f,-1.0f, 1.0f);    // Bottom Right Of The Quad (Front)
    glColor3f(1.0f,1.0f,0.0f);    // Color Yellow
    glVertex3f( 1.0f,-1.0f,-1.0f);    // Top Right Of The Quad (Back)
    glVertex3f(-1.0f,-1.0f,-1.0f);    // Top Left Of The Quad (Back)
    glVertex3f(-1.0f, 1.0f,-1.0f);    // Bottom Left Of The Quad (Back)
    glVertex3f( 1.0f, 1.0f,-1.0f);    // Bottom Right Of The Quad (Back)
    glColor3f(0.0f,0.0f,1.0f);    // Color Blue
    glVertex3f(-1.0f, 1.0f, 1.0f);    // Top Right Of The Quad (Left)
    glVertex3f(-1.0f, 1.0f,-1.0f);    // Top Left Of The Quad (Left)
    glVertex3f(-1.0f,-1.0f,-1.0f);    // Bottom Left Of The Quad (Left)
    glVertex3f(-1.0f,-1.0f, 1.0f);    // Bottom Right Of The Quad (Left)
    glColor3f(1.0f,0.0f,1.0f);    // Color Violet
    glVertex3f( 1.0f, 1.0f,-1.0f);    // Top Right Of The Quad (Right)
    glVertex3f( 1.0f, 1.0f, 1.0f);    // Top Left Of The Quad (Right)
    glVertex3f( 1.0f,-1.0f, 1.0f);    // Bottom Left Of The Quad (Right)
    glVertex3f( 1.0f,-1.0f,-1.0f);    // Bottom Right Of The Quad (Right)
  glEnd();            // End Drawing The Cube
glFlush();
}

Sample 2D Object

void Rectangle(double bottomLeftVertexX, double bottomLeftVertexY, double topRightVertexX, double topRightVertexY)
{
    double x1 = bottomLeftVertexX;
    double y1 = bottomLeftVertexY;
    double x2 = topRightVertexX;
    double y2 = topRightVertexY;


    glBegin(GL_POLYGON);
        glVertex2f(x1, y1);
        glVertex2f(x2, y1);
        glVertex2f(x2, y2);
        glVertex2f(x1, y2);
    glEnd();
}

Rectangle(20.0, 15.0, 40.0, 47.0);