[John Rogers]

I would suggest doing a search for "bicycle spare tire bag" and one of the first to come up is the ones that Walmart sells. The first one on their page for $7.59 looks pretty good to me. Some thoughts: you don't need to worry about aerodynamics as it is mounted under your seat and stuck between your legs and color should be black as where it is mounted it will surely pick up lots of dust and dirt and finally you do not want to draw attention to it so it will not be a target for thieves!