I watched while the tech set up our new power awning. When it came time for him to install the awning into the rail, he used a pair of vice-grips with something covering the jaws top and bottom. May have been either pieces of plastic or thin plywood. At any rate, the vice-grips spread the gripping area and gave him something solid to hook on.
Always wondered how they got that fabric in that small rail. And yes he did add stops to keep it in place.
Like the previous post, drop the awning down and then center it as required.
Good luck and happy RV'ing