What is name of the part/material that goes behind the top section of the rear seat to keep fumes from entering from the trunk?

Will it well if i cut a piece of masonite to fit tight then use chalking, or is that a cheesey way to do it?

I can seem to find the name of the part in the catalogs.

 

I found it! go figure its called a trunk divider. who would have thought!

 

When you install it don't caulk it in, that will look cheesy.