if you can't find one of those boxes, a simple piece of 4" plastic pipe "xx" inches long with a cap on the back side works good, and will only cost a couple of bucks.
make a couple of simple band clamps from thin 1" strap material, and bolt it to the bottom or side of your mail box.
slightly angle it downward at the front to keep the water out, or drill several holes in the bottom of the pipe.
beer