1. First, cut a block to size out of ½ wood trim or, even better, no-rot cellular PVC. I like to size my blocks so that the top and bottom edges line up with full courses of siding because it looks better. Use a jigsaw to cut a hole in the block to fit around the outlet box. Drill and countersink pilot holes for screws in two opposite corners.
2. Hold the block up against the siding's lower, or butt, edge, and, using
the block as a guide, mark the vertical cuts in the siding with a utility knife. Set the block aside, and use the knife to carefully cut away the siding along the top and the marked sides. Don't cut into the builder's felt or house wrap, or the course of siding below the block's bottom edge. Instead, cut a notch in the back of the block's lower edge to fit over the uncut siding.
3. To stop water from getting behind the block at the top, slip a drip cap between the siding and wrap. Squeeze a fat bead of caulk around the perimeter of the cutout, press the block into it, and drive a screw through each pilot hole and into the sheathing. Now attach the fixture to the outlet box. When you're done, you'll have a solid, secure fixture base that also helps protect your walls from water.