A thin plastic such as from a kitchen cutting board cut to a 3/4" strip: you can always put two strips in if you like it a bit stronger. Something like this:
https://www.amazon.com/Kitchen-Essentials-Flexible-Chopping-12x15/dp/B00QPUD0EW/ref=sr_1_6?ie=UTF8&qid=1489776211&sr=8-6&keywords=plastic+cutting+board+sheetsMy sewing machine was able to sew through one layer.
I've tried very large zip ties too but prefer the cutting board.