Those look a lot like just a piece of PVC. I have a lot of junk sitting around from working on the house, that would be a good idea if you could find a piece of pipe that was close to the ID of the seal and then just glue a plug on the end. Since it won't have splines, it will just sit there and not turn. That link made a light bulb go off.There are plugs you can get that slips into the transmission output and keeps the oil contained by the original seal. 4-wheelers sometimes carry them in case the rear drive shaft breaks and they have to limp home on front wheel drive.
You could inert one of these, then still zip-tie a bag on top to prevent it slipping out while you drive:
5 Piece Transmission Stop Off Plug Set
No.J6750 - 5 Piece Transmission Stop Off Plug Setwww.tetools.com