Looked thru the fisher body book and the assembly manual but I can't find any mention of the panel that goes behind the back seat. It's kind of like flimsy masonite and in my car it is falling apart.
What is it called? If I need to make a new one what should it be made of?

 

Trunk divider board is what I have heard it called. You can buy them repopped, but it is easy to make your own if you still have the original. If unusable, use cardboard to make a template than cut a new one out of - get this - masonite! The one I ordered is flimsier than that, kinda like pressed paper-wood...
The insulation pad (jute) goes on the trunk side of the board.

 

I know exactly what you are talking about...hard to find a Toro that has a good one of these if one at all still in it.

They were originally made of asphalt board and over time get dried out and literally crumble even though the material was like 3/8 thick or so.

I do not know of anyone or have ever seen one of these new or reproduced as very little for first gen Toro is reproduced.

What works really well is 3 rubber sheeting. A rubber supply house will have different grades and hence different stiffness so choose one that is reasonably stiff but still plyable. You can piece together your old one for a pattern or tape paper onto the framing behind the seat to make your own pattern. The denseness of the rubber works well for sound deadening that some other materials may not.