I spent many hours both yesterday and today handstitching together the Defender helmet parts with waxed thread. Alas, it turned out to be too small for the plastic form. I cut and sew the leather wet, and when it dries it shrinks, so there is a certain amount of guesswork involved in making the patterns.
For this helmet, the form is important because the leather will be stretched around it to get its smooth shape. Unfortunately I have decided it will be better in the long run to start over again by enlarging the patterns. The other option would be to cut down the form, but that risks destroying it, and rendering all my efforts for naught.