After seeing several examples of this thumb guard posted, I decided to simplify the making of one. The examples of this thumb guard that I have seen all have stitching in the leather, to hold it together. I used "Super Glue".
Paring cut towards thumb with a thumb guard in place.
Leather Thumb Guard
Leather Thumb Guard cutout.
Cutout folded and Supper Glued, held together with paper clips. This seems to hold up very well.