Sealing the backseat aluminum plate from the trunk area.

I just used paintable urethane seam sealer, but I wasn't too concerned because I made a flange around the complete perimeter of the bulkhead to attach it to. In your case, I know they make a fire-rated caulking/sealer for the building industry. Maybe something like that would work. Do you plan on painting over it? If so, that may be another consideration.