It might be too late, but there is a rubber plug device used for oil pans
when the pan's metal plug won't work because the threads are stripped.  It
looks like a rubber baby bottle nipple, and a special tool is inserted
inside it to cause it to become distended.  It is then inserted into the oil
pan opening, whereby the bottom flange rests against the oil pan itself. The
tool is removed, and the plug gets shorter and "fatter", engaging the sides
of the hole.  The same tool must be used to remove the plug when the oil is
to be changed next.

These items were not expensive, and are I believe are not that rare, since
nowadays it is more common for cars to have cast aluminum oil pans that have
threads that strip easily.  They also work with fairly large (5/8 inch)
openings, but the opening must be close to circular.  It's a short cut to
either replacing the pan or re-welding it to replace the blown threads.