A simple little hook that lets you turn an elastic or rubber band into something like a bungee cord. Wrap the band around something and hook it back onto itself. Chain them together for larger bundles.
The band has to be put in place during the print. Check your print preview to determine when to pause it, or print more than one at a time and you can easily put the band in place while the nozzle is over a different one. If you want to use a particularly large rubber band, you might need to scale up the hook a bit.