This project is probably just a big waste of duct tape. I do not know if the post office will even accept duct tape envelopes. So let's begin!
I have shown two standard sizes of envelope: the smaller 3 5/8" x 6 1/2" size and the larger 4 1/8" x 9 3/8" office size. The smaller evelope uses about 3 yards of duct tape and the larger uses about 4 yards.
Hint: The really thin utility grade duct tape works well here.
- Make a sheet 6 1/2" by 9 3/4" for the smaller size and 9 3/8" by 10 7/8" for the larger size.
- Trim as shown in the diagram below. The bottom cuts are not really necessary if you are in a hurry, but make it look more like an old fashion paper envelope and cut down on the bulk.
- Fold on the lines shown in the diagram.
- Tape the two sides shut.
What if you need a different sized envelope? Just wing the proportions!
- The flap is about half the height of the final envelope plus a bit.
- The height for the cut at the bottom is just the flap height minus a bit so that the flap overlaps.
- The indents are a slightly larger bit.
- In this case the a bit is about 1/2".
- The 1/2" at the top just "looked right". Leave it out if you want.