Drop downs are easy sliding needs a fair bit of care to get the alignment perfect and the enclosure must be flat and stay flat or sliders will not be removable or worse bind and not open or close. With your experience I'd say drop down for sure! There are a number of lock systems the easiest are cheap cam locks like these drop down,

http://www.leevalley.com/us/wood/pag...419,41455&ap=2

sliding door lock,
http://www.technologylk.com/crl-chro...htm?src=nextag

Alex