That's my brainstorm for the day: A Tom Bihn lunchbox. A soft-sided, insulated enclosure, maybe with a loop on the inside for securing a can or a small thermos. Roughly the size and shape of the Mac Mini case.
It could even snap into the sides of bags, like the Brain Cell.
I've got bags on the brain: I'm waiting for my birthday so my wife will give me the damn Super Ego she (hopefully) bought for me.