You can sew it.

Find a large-eye needle to thread the adjustment buckle, hold the folded elastic and thread it through the large-eye needle, and pull the adjustment buckle through (like threading double thread with a needle).

Hidden stitch: This is a very practical stitch method that can hide stitches and is often used in areas that are difficult to sew on the reverse side. Usually, hidden needle sewing refers to needle alignment. The needle alignment is to insert the needle in and out of cloth A, then insert the needle in and out of cloth B, and cycle in sequence. The counter needle is often used to complete the final suturing of some return openings.