Okay, now I see that is not a picture of the straps on the couch. Zip ties sound like a good plan. I will have to look at my couch and see how it is constructed.
Heavy web straps or plywood between the frame and cushion. My TH is out camping right now with the wife and kids. I will check on how mine is constructed, I've only had my TH for about 2 weeks. A photo of the couch further away would be better view ...