Finally some updates on this project! I've been very busy with work, this project and some others that I'll share sometime later.
Here's an iteration of the envelope, this time including the letter sheet and both of them using the paper I chose after an exhaustive search. The paper is slightly yellow and has some texture that you can feel (I think this texture will work amazing with the printed one to create an amazing feel and look).
And here's the next iteration. This time I added the Hogwarts crest and the text.
For the Hogwarts crest I searched through the internet until I found a version I could work with. It is the one used in the original prop, not one of the many variations that exist today. I also researched so I could find the exact placement, I didn't the crest to be placed "just there".
The text took a lot more time since I drew every single letter to be exactly like the one in the [printed] hero prop envelope. The color was just green, this iteration wasn't completed before another one appeared.