Exterior lights on new siding, no electrical box to screw into, what to do?
I realize this is a little late, but for those reading this for answers:

Make a 4” round hole in the center. The existing hole in the siding box is around 1-1/8”, so if you have a 1-1/8” hole saw, Nest it in the 4” to stabilize. If you want to save money and not buy a hole saw, trace your octagon box (keep reading) and use tin snips to cut out the hole.

Get yourself an octagon box that has a depth close to, but less than the depth of the siding box. Install an NM clamp (plastic or metal, plastic will work better). Space the box off the back of the box so it is flush to the front, screw it down, and install the light.

Tip: Screws can be your spacer. Make a triangle out of three screws that prevent the box from setting too far back. The secure the box with 2 screws.