Alright, this one took me a lot of time and tries to get it right. Though it still has hiccups here and there.

First, you can add midriff and navel if you want the cutout to be deeper so I left them in the keywords if you want to use them.

The hiccup is that from the back for whatever reason it tries to add a cutout as well. Not sure if because I was using the wrong keywords or just missing keywords but if you don't want the cutout use "back cutout" in your negative prompt. And obviously remember to take out "midriff,navel" and to be safe "cleavage cutout."

I tried to use words like "closed back, solid back" but for whatever reason it always fights me on adding the cutout. If anyone has any ideas/suggestions let me know since I'm still new at this and can make a v2 version.

You can see what I did in the images above.

Sometimes the tube top around the breasts might disappear. If this happens you can add bandeau along with the underboob tag to it and it'll appear again.

Though if you just want the breasts then write "breasts out" and that makes the bandeau disappear.