I cut a smaller piece of leather, skive it to the proper shape, and glue it in between the two sides. I usually glue it to the flesh side of the bottom layer, then fold the top layer over, on top of the assembly. Top layer is an oversized rectangle and inner layer is true to size. Once everyrhknis glued together I cut the top layer to its final saze by following the shape of the inner layer and then I saddle stitch the strap and finish with the edge work.