I read a letter from a woman who sews clothing for many of her (long-distance) relatives. She says that instead of finishing the waistbands of these garments, she sews a buttonhole on the inside of the waistband, pulls both ends of the elastic through it, and secures it with a safety pin. This way her relatives simply try the garment on and sew one line of stitching or just re-pin where they need it! I would leave a little extra elastic on the inside. The buttonhole can be used to add new elastic as needed without cutting your seams. As for your zipper, why not sew it closed using a ladder-stitch by hand or a zig-zag by machine. How about making it into a pocket or adding applique?
If you haven't grown too much you can loop a rubberband through the button hole and then back through itself and secure the button with the rubberband. Don't know what to do about the zippers, though.
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