You don't need a thread, just remove the Exacta mount portion of the Exacta silver adapter and epoxy/screw the cheap F-mount or EOS mount adapter to it. I've done that with old FD lenses, cheap K-mount lenses, they all work on EOS bodies after that. I used $2-5 M42-EOS adapters and drilled holes in the adapter face to put screws into the body of the lens.
Quite secure and reliable in use. Of course, any lens turns into fully manual diaphragm operation at that point.