Herbs in coffee cans, hung with zip ties. Drill a hole in coffee cans & zip tie to railings! You could pretty much mount any container, like a box in this same way. bigger containers may require more zip ties.
container gardening ideas Idea Box by 360 Sod (Donna Dixson)
radmegan: How to make Molded Concrete Planters - use plastic containers to shape the pots and smaller plastic containers inside larger ones (ideally, leaving about a half an inch of space between the two.