Coming from someone who is (I think) a D cup, I've run into similar problems. I have a Tri-Top from Underworks, which doesn't work perfectly, but definitely does enough that I feel a lot more confident with my passing. When your a larger size, any flattening you can get with safe methods is a great start..I'd suggest considering a binder of some sort, even if it doesn't work as perfectly as it would for someone smaller.