i like FL and BL as well.
if you have coach sommer's book, he has a nice section on straight body work. one of these exercises is called simply "body levers." basically you get into a candlestick pose or plough pose but with your feet pointing straight up into the air. you should grab a squat rack or a pole or something with your hands up behind your head. now lower your legs to the floor in a negative fashion, keep the straight body position, and raise the back up. i looks like you're descending into a front lever and then pulling back up to inversion. by doing it on the floor, you can take out the arm focus and work on your core control more. you should make it hard enough that you can get about 5. if it's too hard pull your legs in when you ascend, or straddle or whatever. if it's too easy, strap a little weight to your ankles. just maintain the straight body no matter what you pick
obviously you should try a lot of different compound moves to strengthen the core. i just find that if i had to pick one exercise that i knew would make my midsection hurt the next day, this would be it.