You can also do hashes:

You use a GPU to do N parallel chains of M length. You then encrypt the seeds for the chains using the previous chain’s ending value as the key for the seed.

The seed of the first chain is your starting point; the ending of the last chain encrypts your key.

Anyone seeking to “unlock” that key must do the hashing sequentially — meaning that you can use a cluster to get ratios like 1,000,000:1 between generation and unlocking.

For reference, there’s around 500,000 minutes in a year.