Ask HN: Are there any good solutions for public audit logging?

I'm looking for a service, blockchain-based or not, that would allow me to publicly (and ideally irrevocably) log audit-related information, i.e. put a blob into a database with a unique ID/URL and allow anyone in possession of the ID/URL to verify that the data was put there (and when). Any recommendations? It should be cheap (< 0.01 $ / transaction ideally) and fast (confirm a transaction in a few seconds) so I think using something like Bitcoin or Ethereum is out of the question.