Get rid of the box.
Measure the back of the fixture, and cut a piece of plywood that size.
Mount the plywood where you want the fixture, and stub a wire out.
Have the siding guys J-channel around the plywood, and bend a piece of facior(spelling)metal the size of the plywood to cover the plywood, make sure they leave the wire stubed out.
Mount your fixture when they are done. Very clean looking installation.
Dnk...
