Many ideas have already been shared, and I know someone who was looking for options already must have seen an option they like, but since we are still sharing ideas on it, I like to be creative and share another way you could dispose a steel plate seed backup.

Throw it into the sea or the ocean if you stay close to one, it will take a 1000 years before it will be discovered, and even if it is found at the bottom of the sea, it is totally random, and the people who discover it may never have a clue what it is.