I have created a visio with a box that can be cut out (once you print out this pdf on cardstock or just glue paper to a cardstock holder).
I saw that they use a tool just like this in the post office to test our letters. If it doesn't fly through, they ask for more postage! It fails as a letter and needs Package rate postage (roughly double!).
Many of us send blocks in the mail and I'm am so irritated at how many have come back to me lately.
I hope this helps you! Let me know if there is a way to improve the instructions and I'll edit it with your input!
Cut out Bolded Box in this PDF to test your envelopes