Is the wall cavity on either side of the existing panel empty? If so you can recess the ATS into the adjacent cavity and put nipples through the studs between the two boxes. The ATS will stick out from the wall a bit but it results in a clean installation.
This is the exact same way i do all my installs. i use a 2" piece of continuous threaded plastic nipple (schedule 80) between boxes to eliminate bonding issues. It makes for a very clean install. Just eliminate the prewired steel flex pigtail all together.