I don't really think that my core is being worked enough when performing the compound lifts of strong lifts, as the rest of my body has caught up while my stomach and waist are still really flabby and have no real firmness comparatively. I'm not asking "what exercise do I do to reduce stomach fat", I'm asking if I can add in some cable crunches and another auxiliary exercise to strengthen and firm my core for other lifts and if so what would you suggest? 

As I understand exercises like sit ups and laying crunches and other variations(leg raises) are limited mostly by the inability to use weight above your body weight effectively so weighted exercises for abs/core are recommended.

Thanks!