A new thought: if you need flares, you'll need a cellphone, whistle, et. al. How about a large nylon back flap pouch with separate pockets, zipped from shoulder to waist on the back of the PFD, and velcroed with a buckle on the other shoulder back. When in need, release buckle, swing all your supplies for safety to your front side, velcro it there, and then select your flare, cell phone, whistle, lip balm, etc at convenient position.
I am not a fan of paddling with water high on my back for stability and butt comfort. 3 more pounds on my derriere could finish me off.
Flare guns and shells are much easier to use when in trouble than popping off flares with cold hands. Their reliability is far better also.