You msy have seen thus site already.
I think this style might sit well against a fkst or smaller bust. If it has a shelf bra you could cut that out.
https://www.swimsuitsforall.com/Riff-Mesh-HighNeck-Tankini#rrec=trueOtherwise an actual proper sport swimsuit that is designed to push you flat might be a better option.
I have a zip front version. ..that can hold a prosthesis if you want. But racing suits push you flat anyway.
http://www.swimweargalore.com.au/speedo?gender=women&size=18