i know this sounds lame, but
i know this sounds lame, but a fanny pack works great. it's super convenient, you can hold water, food, cell phone, wallet, etc. all in a way that is always accessible on AND off the bike. An unexpected benefit I've found is that I am FASTER when I put stuff on my belly. When I ride with a 'backpack' on my belly I am noticeably faster. I think that filling the void that would normally be left between my thighs and chest is good for aerodynamics. So keep the fanny pack in mind. It's cheap and multipurpose. You can use it for hiking or anything else you want, too!