Or you could use magnetic reed switches like http://www.ebay.co.uk/itm/White-Door-Window-Contacts-Magnetic-Reed-Switch-Sensor-/350647348537#vi-content
Or you could use magnetic reed switches like http://www.ebay.co.uk/itm/White-Door-Window-Contacts-Magnetic-Reed-Switch-Sensor-/350647348537#vi-content