The dank South Boston warehouse held a chill last weekend when the flatbed arrived, a 20-foot-long landscaping trailer with a wood plank floor stained by dirt. Around the flatbed gathered almost a dozen people — predominantly men from the surrounding blocks — wearing winter coats and work gloves. The job ahead was daunting: transforming the flatbed into a parade float with seven faux cannons. From each cannon would flow a different color of the rainbow — fabric draped over plastic piping — landing in a pot of gold. Randy Foster and Steve Martin had done this before. They built a...