If you can't find a wood plaque with those exact dimensions at Dollar Tree, they sell 7-inch-by-9-inch Crafter's Square DIY Wood Plaques that you can embellish with smaller stencils.

To craft this design, just paint scrap wood square in a color of your choice, and use E6000 to glue all of your smaller metal objects into place on top. You will need to drill the horseshoe in place.