Use a thin sheet of paper, that has a "toothy" surface and cut a freehand section just a little larger than the pattern you need to fill.
Use a double sided piece of tape or two to locate your paper, roughly centered on the infill section and using a good, #2B pencil, make a light rubbing with the side of the graphite lead of your pencil, until you have a crisp sharp outline of the missing leather panel from which to build a replacement insert.
Nothing too hard or expensive, involved I this except, perhaps, the outlay of money for Pliobond 25 adhesive for laying it down.
Cheers