I figured I'd reply here since it's easier to show the pics I've taken.
Here are the panels I've got, they're from a '78 244 so they originally had the round lights, wide grille etc.
The openings are 17cm high x 24cm wide, at the widest point. This size and shape accomodates the back of the large rectangular lights.
When the narrow lights (round, or square lights) are used, they bolt to the four captive nuts that are built into the panel.
When the wide lights (rectangular, or California) are used, they bolt to the two outer captive nuts in the panel, and to two clip-in captive nuts that are fitted to the square holes next to the inner corners of the opening.
Here are the nuts: