It's not actually the bar and hook that needs replaced. It's the material. I do have a new hook and bar though. The part of the waistband that has the hook on it has pulled away from the material and no longer holds any tension when "hooked". I need to add some material (which I have since I've had to hem the pants) to the waistband (underneath side) so the hook has something to attach to, but I don't know how to do that without any stitching showing on the outside of the waistband. Any ideas?

Sue, April 2016

Cut your material slightly larger that the waist band. About 5 mm (0.25 inch) on all sides.
Fold it's sides under so that the edges are hiden.
Stitch around through the actual stitches on 3 sides. The stitching will get hiden in plain view onto the original stitching. Make sure that it's colour matches, or is slightly darker than, the original stitching.

For the last side, use a "spiral" stitch where the needle goes in next to the patch and out through it. Not eastethic, but safely hiden from view.

You may want to have double thickness to get it stronger or stiffer. You may attach the bar or hook beforehand.

Electro, May 2016