Horizontal divisions of columbaria shall be of reinforced concrete of at least 50 millimeters thickness. Vertical divisions may be of concrete hollow blocks of at least 100 millimeters thickness, in which case, cement mortar shall be applied evenly to present a non-porous surface. Minimum dimensions of individual cinerarium shall be 300 millimeters by 300 millimeters by 460 millimeters. Cinerary remains shall be placed inside tightly sealed urns.