Put standard rubber band placement on each four-bar, then add a constant amount of stretch to it. You may do it backward at first (guilty…) but just play around with it. You also want bands on both sides of each C-channel so they do not bend.
Standard rubber band placement would be a straight rubber band one bar to the other at the correct angle to assist it. It’s very hard to explain without a diagram, but I can’t make one right now. The picture should suffice.
The zip ties keep the rubber bands from getting stuck on the top four-bar when it comes down.