layers

seriously, layers are the only thing i can think of right now - i'm a kind of bigchested dude, and when i can't bind for reasons, i just wear a lot of layers - of course it doesn't give the same effect as a "proper binder", and it may be pretty warm, but yeah
maybe a tight-ish tanktop, with a t-shirt on top, then a hoodie over, and a zip-zap(idk what they're called - kind of like a cardigan??) hoodie over that hoodie