first of all, heavy compound movements require stabilization from the core. Squats and deadlifts espechally recruits the abs for stabilization.
Other than that, you might want to try some planks. try to make a search for abs, as im sure you will find alot.
1.) Weighted Situps
2.) Crunches (lay on the ground, feet up, put your hands behind your ear, look into the ceiling, and at the crunch part hold for 2 seconds then lower
3.)Supermans
4.) Roll-backs
5.) Planks