Commercial Electric 4 in. White Mounting Plate and Mounting Screws for Commercial Electric LED Strip Lights Covers Standard Junction Box 54657101 - The Home Depot
Sigma Engineered Solutions Metal Weatherproof New Work Round Electrical Box in the Electrical Boxes department at Lowes.com
Cantex 4 in. Round Junction Box R5133690 - The Home Depot
4" Square Electrical Junction Box to Round Light Fixture Adapter by lpalbers - Thingiverse