Yeah, the triangle bags that are on the market are not really designed for quick removal. Also, you can't have anything too thick on the top tube, between your legs. There are pretty large, waterproof handlebar bags too, if you don't mind that, such as the Ortlieb bag, it's pretty roomy. But it'll affect your steering somewhat when loaded heavy. Finally, there are pretty large saddle bags too.
Have a look at
Porcelain Rocket. A bit pricey, but they will make almost any custom bag for you. Their Big Dummy triangle bags are popular.