When starting to take estrogen, I used really tight sports bra to hide my growth from the people who didn't know.
Can't you use that?
I am 32C (or just go measured for the wrong bra somehow) or 32B (I think it might be more accurate even though I was measured) and no one seems to notice if I wear quite a big shirt and a notebook.. but I guess they don't expect it either.
Then again it's ridiculous to walk with a notepad all the time when I go around at the village (I don't really when going out, outside I can be me

)
My grandma told me "Maybe put bandages and wrap them tightly so you can walk freely in the village?", But we didn't try it.
Something must be weird when I read people with B are having hard time with a binder and no one notices mine and they feel small O_o
Edit: Oh! I think black helps for hiding even better.