A canopy does what you want but I haven't seen one for an exterior wall mount, they are usually for interior ceiling mount.
The box you have there should be an exterior one, not a plain metal one like that, it should be recessed in the wall and there should be a cable or conduit coming through the wall, entering the box, and attached to the entry hole.
There are other kinds of exterior boxes that are intended for surface mounting, will look better, and will interface nicely with appropriate fixtures meant to be mounted on them. They don't look as nice as a flush mount but they look a lot nicer than a fixture hovering a half inch off the wall.
In every case however you should have a cable or conduit coming through the wall.
Edit adding this thought after seeing your link to the kit meant for vinyl siding.
If you are somewhat handy with a jigsaw and a router you could make something much nicer by cutting some 1/2 inch composite material to the shape of your lamp base, routing the edges to match the lamp base, and cutting a hole for the box in it.
- Option 1 - Buy a 1/2 inch thick piece of white composite trim, like a 12x12 inch board. Trace your lamp base onto it. Add a quarter inch all around, cut it out and route the top edge to a nice shape. Then paint it black or a color to match the lamp or the wall or the pointing.
- Option 2- Use Marine Board HDPE. Similar process to Option 1, more expensive, but you can buy it black so if you want it black you don't have to paint it.