I made a trailer "hitch" for my E3 SX from two aluminum flat bars purchased from Home Depot (this item: http://www.homedepot.com/p/Crown-Bol...4690/100337476)
First I bent a 7 1/2 inch 90 degree bend in each bar. Then I drilled two holes so that each bar could be attached to the two anchor points on each side of the top portion of the rear axle housing.
Then I bend another 90 degree angle at 2 5/8 inches from the first bend so that each bar turned toward the middle of the rear of the bike, and the remainder of the bar stuck out the back end. I cut the remainder so that it didn't stick out past the basket.
Then I drilled a hole in the end of both bars to attach the bike trailer receiver. I also drilled a hole near the edge of the second bend to use a screw to keep the two bars pressed together if I remove the trail hitch for any reason.
click on the pictures to see a larger version of each
From left side:
From right side: