Sigma Engineered Solutions Gray Metal Weatherproof New Work Standard Box Extender Exterior Electrical Box in the Electrical Boxes department at Lowes.com
Cantex 4 in. Round Junction Box R5133690 - The Home Depot
RAB Lighting VXC Weatherproof Round Box with No Cover, Aluminum, 1/2" Hole Size, Natural: Electrical Boxes: Amazon.com: Tools & Home Improvement