Mat I'm not a believer in spot loosing like doing more ab work. More ab work will build your abs and tighten them , as you know. The only way I have found was some sort of aerobics. The skin might be holding on to some fat tissue (just a little) and you have to burn that off just like people loosing weight.
I have done this with not wanting to loose weight but to loose some unwanted fat tissue around y waist. However this I have done as an older guy.
I don't recommend , but I used a very small amount of a diuretic to loose that last bit of water. BUT this was pre-show and stage.
Currently I'm riding the stationary bike AFTER my workout.