Another litter box, filled with Dr. Elsey's Cat Attract litter, might help:
http://www.preciouscat.com/
Rather than a pheromone collar, I recommend trying a Feliway diffuser or 2 in case it is stress-related:
http://www.feliway.com/us
Along with what growler mentioned, a diet change to something better (and wet), whether or not that has any effect on the poop issue, is a great idea. Better for her overall health, which may have a positive effect on her ability to handle stressful situations, such as when you're gone.