I followed Shadow Catcher's method and attached a PVC awning rail with 3M VHB tape and a screw on each end of the awning rail for extra security, as the back surface of the trailer bowed away from the awning rail slightly.
I sewed an awning with welt from the same supplier -- much more aesthetically pleasing and easier to put up than the plastic tarp we used last year (of course, we only got the trailer a week before that trip last year.)

The camo tent flap with screen that I created for last year's trip (attaches to inner side and top edges with velcro) is for sale, in case anyone with a 7' wide, 6-1/2' tall trailer is interested. We now have a back wall with a window, so we won't be able to use it anymore.