Front Drive Shaft Slip Yoke Grease Retaining Cap(?) (1 Viewer)

[kevin in okinawa]

where I have the yellow circle positioned at the end of the slip yoke, I suspect that there is supposed to be a cap that keeps the grease inside the end of the cylinder...mine doesn't have one and sadly it appears they are NLA. Any spares floating around out there or ideas? could a 3-d printed thing work instead?