You could probably use the foam sheet boards, just cut a few for thickness (4x8 sheets)
Would this be for the space the AC unit was? Against the exterior metal? 

Trying to think how to tie in with the existing (I had some pictures that showed  the questions much better 😕). Had just been brainstorming and thought to add insulation (paper back or ??) and then put a vapor barrier over that, trying to seal to the plastic backed insulation that covers the rest of the wall.