I often make my own envelopes to mail special cards, store flat items in my studio, or just because I love using templates, cutting and folding. We paper artists are a strange bunch. Any catalogs or magazines with full spreads of interesting pictures are fodder. I especially like the matte finish heavier publications you see sometimes.
You can take any envelope you have that is a size and shape you like, and just open all the flaps to have a template. Or you can extend or contract any of the dimensions and make your own template from card stock, copy paper, or even layout bond. The larger template below is on layout bond and has extra dimensions I built in to accommodate thicker contents.
It helps to know what the legal limit size limits of the USPS are if you are going to be sending your envelopes in the mail. Too small, too large, or too thick costs extra. At some point years ago I got my hands on this official USPS guide. I have found it very useful.
Over time I have saved all sorts of templates for unusual sized envelopes and boxes.
These little Glassine beauties are very old from my fathers stamp collection supplies. You can still find versions of them available today.
Holiday paper art found on Houzz.
Merry Christmas Everyone!