lower abs cannot be worked separately than any other section of the abs. so dont worry about trying to do something that cannot be done. if you think you feel it more down there it is cause you are feeling the hip flexors working and think it is the lower abs. great ab exercises are planks (side planks too), dragon flags (or as someone called them, leg raises on decline bench), pikes (also known as jackknife) with feet on swiss ball, standing cable crunches, farmers and yoke walks, fallouts or rollouts, and pavlov press. heavy squats, overheads and deads too as you have to contract them hard to stabilize the body.