gto19.....Unless someone has a better idea, there is really no way to fix the stretched slide cover unless you remove it and sew the loop in the one end a little tighter. That cover is very easy to remove if you do this
Carefree Slide Topper Repair/Replace. If you scroll down in the above post you'll see how to easily remove the big slide cover. After removal you can cut off the cuff that holds the plastic tubing at the rofline and sew a new cuff onto the material. When you sew the new cuff, make it tighter (shorter) at the end tah is sagging. I had an upholstery shop resew mine in a couple of minutes for about $20.00.