Filling a pipe, while setting on a parkbench... from a rolled up ziplock bag is just begging for the police to walk up and start asking questions.
I just pack a few pipes and carry them in a small pipe bag. If I am going out with just one pipe to smoke multiple bowls from the same pipe, I put the tobacco in a small square/rectangle tin. They fit in the breast pocket of a sports coat very well. Or, in the breast pocket of my overalls if I am out working in the field.
But, if we are getting creative here, my ears are the perfect size for one bowls worth of tobacco. Plus, if I keep tobacco in my ears, I don't have to listen to the loud stereos on the motorcycles riding by. Lately, we've been inundated by loud stereos on bikes. I don't get it. Why not just wear earbuds? I mean... if they are going to turn it up so loud anyways, it doesn't help with being able to hear traffic any better.