This part of the restoration was unexpected. When I discovered the factory speakers screwed into this space on an open-air piece of stamped sheet metal, the course of action was clear: New speakers:
The stockers sounded just OK with the Infinity sub I had behind the seat, but that went bye-bye along with the door a couple years ago. Paired up with a JL Audio HO-110 sub, these should take it into the realm of true Hi-Fi. Only two pieces of the old system remain, so a new head unit and amp for these speakers is apparently called for now.