If you already have a sling...I would lay it out....folding it in half long ways and tuck in the elbow end of it to make a rectangle....measure it top to bottom and side to side...then double those measurements and add one inch in height and for 5 inches to the width for just in case :wink:
Then I would find a pretty piece of fabric that would match several outfits... like a multi colored batik....pin the fabric wrong side up to the outside of your sling leaving the extra hang over the tops and sides...mark and pin a dart in the elbow part and then mark where you need to trim around the top and wrist opening.... remove the fabric and sew the dart and trim along your markings...then re pin it in place right side out and bind around all of the edges.
You may have to remove some of the webbing and re sew it on again.