My approach:
Two linear actuators (one on either side - the short sides, it's pretty much rectangular I suppose) and four guide rails (telescopic if you don't have the space above), each one preferably able to lift around 15 lbs by itself. Then you're operating them at about half the rated power which should keep them nice and cool and work long - but do weigh the whole assembly before choosing your actuators.
Add limit switches on both ends of the movement to stop it from going too far.
Another approach would be two winches instead of the linear actuators, but I suspect those to be more failure prone.