RACO® 150 Old Work Bracket Octagon Box, 4" x 1-1/2" – toolboxsupply.com
2 Pcs, 4 In. Octagon Box for Cut In Old Work, 1-1/2 In. Deep, (2) 1/2 In. Side Knockouts; (3) 1/2 In. & (2) 3/4 In. Bottom Knockouts, (4) Threaded Mounting Tabs, .0625 Galvanized Steel - Walmart.com