Rust-Free Mailbox Mounting Bracket, Aluminum, Large (fits 21&rdquo; L x 8&rdquo; W mailboxes) PLEASE READ: Fits large mailbox sized 21 length x 8 width and posts of 4" - 6" widths. Easy installation with pre-drilled holes; instructions included Made in the U.S.A. with mill-finish and factory bent Rust-free aluminum, resists rot or rust Bulk discount pricing available for purchases of 50 units or more. Scroll down to Special Offers and Product Promotions section for more details.