Going to put this in a new post since I have some better information now. The first picture I posted is what is apparently called the Bellhousing Inspection Cover or some variation of that. Omix-Ada makes one and 4WD Hardware carries it. Part Number: 16917.01
Summit Racing offers what they call a universal transmission dust cover without a starter cut out that they say will work for the TH400. It is plastic so I'm thinking it could probably be made to work with some modification?