I suggest, as I stated in my other post, looking at the LuLuLemon pockets - specifically, having the pocket be a webbing/spandex material which cinches the contents of the pocket to the body such that it doesnt flop about and also secures the contents into the pocket.

I like the horizontal, but maybe a 15 degree angle would be best, however, do the horiz pockets get in the way when you sit?