Last year I purchased a Big Agnus awning
with is supposed to be used with it's attached straps on the side of a car (off the luggage rack). I cut off the straps and attached an awning rope, which I purchased at Sailrite. Just do a search for Sailrite, and then search their website for awning rope. Several kinds come up. I used the vinyl 3/8".
It was alittle tough to work with, but in the end it does work great! I used an old singer machine, which I moved manually, with thick thread. I had also purchased a roll of double sided sticky tape which helped hold the awning rope in place because needles won't work. The stiff vinyl rope makes it easier to slide into the rail.
They also have softer ropes, which may be easier. Since you buy them buy the foot, just measure the rail on your trailer.