Stephanie....You can buy an attachment at most floral stores such as Hobby Lobby, Michaels, Trees & Trends. Use a floral brick, add flowers and greenery. Place in the shape of a swag on both sides and fuller on top. Hope this helps
Scroll down to the bottom of the page:
Here ya go
You can make the swag for over the top using this method: https://www.hometalk.com/44253419/decorate-your-mailbox-for-valentine-s-day
Then, instead of using the zip ties to hook it beneath the mailbox, use zip ties around the brick mailbox to secure the "legs" of the monument saddle.
There is a great post here on making mailbox swag. You can use outdoor adhesive strips/hooks to hold the swag in place for extra security.
Here's one idea: https://www.hometalk.com/30528496/mailbox-swag-for-fall
check this pin https://www.pinterest.com/pin/467318898835556414/
This is a great tutorial for Valentine’s Day mailbox swag: http://celebrateanddecorate.com/a-valentine-mailbox-decoration/
This is also a fun tutorial: https://www.misskopykat.com/2014/12/curly-deco-mesh-mailbox-topper.html?m=1