Steel Plate can be acquired with Strong Rope in the Cliffs region of the main Island.
This can be used in the following recipes:
Where to Find
In the Cliffs region of the Island, use 2x Strong Rope to climb up the cliffs, then 2x Strong Rope again to climb down the cliffs on the far left side behind the palm trees there.
On the next screen you will see a rock maze. Hidden beneath one of the rocks is the Steel Plate. The way to solve the puzzle is explained in the image.
Steel Plate will re-generate every hour, so come back often!