Well, OK. I was thinking of a rubberized foam. They sell this at a rubber supply place that is local to me in different thicknesses. I have used it many times in other cars that were being fully restored by a shop I used to work for. I do not believe it has any absorbent properties.
I guess if it is being recommended against, I'll have to think of another way to go. I'm not sure how I'll be able to get the fiberglass patch to hold it's shape even if I use wire mesh.

I started making a template that will be used when I make the patch panel that will be installed.



This car in general is turning into a far bigger issue than I was hoping. I'm finding rust in all sorts of inconvenient locations.
One example of this is the sunroof tray. The rear corners are completely gone. I will need to remove the tray in order to deal with it.



I'm having to seriously consider whether all this is actually worth my time. I was hoping to have the car on the road by early Summer. But, that is getting further away from being possible. Every time I find something like this it takes time and money (and morale) away from dealing with the issues I already knew the car had.