Black Nylon Vadney Clip. Insert into the extrusion on a rub rail or windshield. Screw in a 3/8" stud screw to tighten and set the Vadney clip in the extrusion. In the future should your canvas shrink or stretch, simply loosen the stud screw and move the clip.
This clip will fit in a 3/8" plus deep track.
Sold each.