The first step is to mount the ends with bar the off once the panels are off. Then you can put the panel next to the mount and see where to start making the hole.
Picture of the hole I made. Took alittle too much on the top off it though. It takes some time to take alittle off and then check fitment then repeat till you are happy with it.
Picture of the mount without the bar on. And yes I put some second skin insulation in awhile back and thats why it wasn't that hard for me too remove the panels the second time around.