Since it's not under pressure it doesn't need to be a solid plug. Years ago when a drain plug was striped I just used a rubber plug like you sometimes find in a thermos bottle. You have to find one small enough to fit the hole, screw the little handle down to expand it then flip it closed. It will expand enough to block it completely, never had one fall out, had one in my old '48 Dodge all through high school and beyond.
They make a rubber drain plug that expands when you tighten it down. I used one on a stripped Mecedes oil pan for several years until I sold it. Bought it at a local auto parts store.