I just traced the ring from the heater motor on a piece of sheet metal I had lying around, Cut it and drilled a few holes. Same with the cover for the heater heater hose/heater core.
The heater hose block off is the same outside at the rectangular heater hose grommet plate but without the two holes for the heater core nipples. So the block off there will just have a sheet metal screw in the middle of a rectangular plate.