How you hang on to the cases depends on exactly what you are working with and how big they are. Where possible I put the panel down on the spoilboard and secure it with clamps on the edges and screw the center of the cutouts down to the spoilboard. Work from the back so the panel lays flat.
Here is one I did a while back. Using a combination of edge clamps and screws/washers and moving the screws around as needed, I was able to keep the panel well anchored to the spoilboard as I was working.