I'm a 46 H. Oi. I was thinking for a long time about making my own bras but after realizing that 1. there aren't patterns in my cup size and 2. there's very little room for error in sewing bras, I'm going with the store bought stuff.
I have to second, third? the bra fitting recommendation. All my bras are 44 DD because that's what I could find and stuff myself into. Then I saw an awesome documentary on large breasts and they sent the women for fittings to a toronto store -- Secrets from your sister
. The difference between "what I can find and afford" and fitted bras was amazing, mind-boggling. So last month I headed out to Secrets and got fitted. The saleswoman (Fawna) was fabulous, and not only found me a well fitting bra, she taught me what to look for in a well fitting bra.
I really think that going to a store like Secrets and getting fitted should be the first step in figuring out how to deal with building a better bra. I mean, what's the point in making a bra that matches the other bras you may have that most likely don't fit?