Floodlight mounting to 4 in junction box

I just received a floodlight which seems really nice. However its mounted on a fecia and would need a junction box behind it for wire connections. The floodlight is 5 inch diameter. Every round junction box ive ever seen is 4 inches. Now, this isnt a big deal if the junction box is flush with the exterior of the wall like they show in heir set up, BUT if the box is miunted directly to the floodlight like it would need to be on a fecia or eve then this causes an issuse.

The solution is to make the floodlight mouting diameter 4 inches like every other manufacturer does for this very reason.