What a fun project! Used sz 8 bolts, nuts, and washers. When picking the chain to use, keep in mind that each of these cans, when filled, will weigh approx 10 lbs... So the top chains will be holding up about 30 lbs. don't get too light-weight of a chain.
DIY Herb Garden: Most herbs love sun and are made for bright kitchens. Line your window sill with small pots, or purchase a long plastic planter made for sills. Or, experiment with various containers, like colanders or antique watering cans.