The envelope seems like a lot of work for an unreliable result.
Here's a much simpler and more reliable method:
E-mail the data to yourself.
The mail server's site (e.g. Google's gmail) could guarantee that you had no opportunity to make changes to their data or metadata.
Ideas in of themselves are not intellectual property. What is intellectual property is the expression of your ideas. In other words, the prose text that incorporates them. In fact, the moment you write something is already copyright to you.
Frankly it is a complete waste of time to attempt to hang on to your ideas as belonging to yourself. If for no other ...