I don't know if you're familiar with it (because likely it was either omfg your childhood or it wasn't, by and large) but a group of friends and I are doing a Yu Yu Hakusho group crossplay, and in planning I was struck by how nearly every costume we're doing is along the lines of Baby's First Crossplay.
Yusuke is a baby-faced fourteen year old cutie punk who wears girl jeans and things that can be found in a thrift store, Hiei is 4'11" to 5'3" tops and would look almost exactly like his moetastic sister if not for the hair and the fact that he's composed of 90% PURE HATE, and Kurama has luscious long hair and eyelashes by Revlon, wears a magenta school uniform when he's not wearing flowing tunics that resemble qipao, fights with pretty flowers, and is mistaken for a woman both by fans and in the manga.
(And then there's our guy friend cosplaying Mukuro, whose first big reveal was Surprise Everyone I'm A Lady, but that's less relevant to you.)
At any rate, all three of the guys mentioned have at least a few recognizable canon outfits with loose shirts. When I cosplay Hiei, I just use the "two one-size-smaller sports bras" method in combination with a loose-fitting shirt, and it works fine. (I'm a C-cup, since that's probably relevant to whether or not it'll work for you.)