Veradek Brooklyn Rectangular Window Box Wall/Railing Planter (9" H x 10" W x 36" L, Black), Single wall molded design, made from high-grade polyethylene composite By Brand Veradek - Walmart.com
Countryside Plastic Flower Box Planters or Liners
11 Best Window Boxes for Summer 2018 - Window Boxes and Planters for Flowers
Bloem 36-in W x 6.7-in H Living Green Recycled Plastic Window Box in the Pots & Planters department at Lowes.com