If Joseph Smith had created the gold plates by himself it might have been easy for him to have created them using materials which are out of place in ancient America. It also could have been easy for him to have created them using materials which would not be conducive to lasting for centuries or being engraved on.
However Joseph seems to have avoided these issues based on the witness statements which suggest the plates were made of the alloy tumbaga. This fits well all the criteria that the plates would need:
If the plates were made of tumbaga it would explain why witness statements said they were:
What are the odds that Joseph would not make any of these easy errors when creating the plates?