These adjustable bungee cords are the bees knees.  You can cut a bunch of 6mm bungee cord to 24-36" and slide the hooks to what ever length you need.  I use these things for everything, 

Will they work as well as a metal bungee cord, likely not, but I haven't broken mine yet and use them for all sorts of little items.

my favorite is the in line hood (hole down the backside.  It holds in line with the tension and I find it stronger due to the width, and easier to adjust. do use both because the other is much easier to print.