Ask HN: Low cost source code escrow

I am working out a contract with my customer and 1 of the items they are asking for is source code escrow. I am happy to provide this but we both agree that involving a real source code escrow company will both cost too much and require a non-trivial amount of recurring work.

Are there any low cost and low effort source code escrow services? or Alternately:

Is there a way to create a honor based source code system where the customer has the ability to access the code but I would get notified if they did?

One example would be to put the code into a shared dropbox folder and send them the link, but if they did access it I would get notified. The drawback here is I might be able to delete the content or close the account without their knowledge.