I don’t have a lot of experience in building armor myself, but I reached out to a friend who’s been into extremely enthusiastic for several decades and happens to be a woman. Her recommendation is a globose breastplate with padding for additional support. Something like these:
It is possible that one made off a peg suit you, but more likely that you would need one custom made. Regardless you’d be going to a smooth deflective curve such as in the illustrations above. Plate armor like this is actually quite roomy in order to allow movement, so there’ll be plenty of room to add supportive padding.
Globose breastplates are held on with a harness, so with firm padding should be able to restrain even the mightiest bosom. Similar armoring techniques were often used when making custom suits for rotund nobles, Henry VIII of England armors show a gradually increasing girth throughout his life.
(Photo by Chuck, King Henry VIII’s armor in the Tower of London’s Royal Armouries.)