Gilbert & Bennett 24" Adjustable and Expandable Black Flower Box Holder. Made of vinyl coated steel, this durable holder can be used with a variety of flower box sizes as it expands from 24" to 36". It also comes with adjustable hanging brackets so it can be used on deck railings from 2" to 6" wide. Customize your front porch, backyard deck, or back deck railings with this versatile holder.