With the bezel from each light removed, you could put some grease pen grease on the circular edge of the light fixture and mount the panel right up against it, then remove the panel. The grease will transfer where the light was pressed up against the back of the panel and you can cut there.

Also, you could lay the old bad panel over the top of the new one, clamp them together and use the old one to trace or even as a router jig to cut the new one.