My brother bought me some slip on shoes. And he bought me a size 11 when I'm a size 9.5. When I stand on my toes the heel drops off my foot. I decided to add a strip of fabric to hold the shoe onto my foot.
For the fabric strip I used some stretchy nylon cut into two 6" by 3.5" rectangles. We'll glue the fabric strip into the shoe using E6000, which you can buy on Amazon.
The main challenge is holding the fabric in the shoe while the glue dries. To help hold the fabric I cut out two pieces of pasta box (a cereal box would work too) to make supports for the rectangles. Then I taped the fabric onto the paper supports with scotch tape, pulling the fabric so that it's tight on the paper. Later, when you push the fabric into the shoe, it won't slide off the support or bunch up.
I applied a 1/8" line of glue on three edges of the fabric, about a quarter inch from the edge. Bending the paper support, I inserted it into the shoe. I pushed some crumpled paper in the shoe to apply a little more clamping pressure. Do whatever is necessary to get the two pieces of fabric to hold while the glue dries. Let the glue dry in a well ventilated area. I waited several days before trying to use the shoes.
I originally planned to cut up an old sock to cover the black stretchy fabric. In the end I didn't bother because I figure the black stuff doesn't look too bad. The bond feels pretty strong. I'll go running in these shoes and see how long it lasts.