Well they make sew on plastic rod that is used for "rope & pole" awnings sold for vintage trailers. Both Marti's awnings and Vintage Trailer are good sources.
There are bag awnings that hang in a bag from the rail all the time, more common on pop-up campers but have been used on FGRV's. Available at RV stores or Camping World.
You will probably find that those fiberglass reflective rods for marking a drive way will fit and they can be sewn into the edge of a tarp or homemade awning
There are also these made from aluminum tube 1/4 inch, some paracord and a large washer. I'm going to be giving these a try with a 10 x 12 tarp.
Make your own Awning Track Hangers
I'm going to go a bit longer on the aluminum tube to spread the load out more. I'm also going to be using a tarp longer than the trailer and using a tarp pole at the front and rear of the camper to take some load and allow a longer tarp than the rail.
Only around 7 foot of rail, camper is under 10 ft. so 12 foot nylon tarp will extend a foot past the front and rear. I can use a pole and guy lines to pull in both directions at the trailer side. Three poles on the outer edge. Hopefully it all works. Or works well enough until we decide to break down and buy a cute looking one with pretty colors and a valance from Marti's
I think most would suggest taking tarp down in the event of storms or high wind so that is something to think about in terms of ease of put up and tear down. I may snag a smaller tarp for bad weather at some point. Sort of like reducing the sails on a sail boat in high wind. For shade use a 12x10 and for storms use a 6x7 just so that there is some canopy for the door.