Here's the shock cord I use:

https://www.amazon.com/gp/product/B0084EFED2/

Nice and springy.

I'm using it in both things like the string crossbow https://www.thingiverse.com/thing:4797080 and for other items like the Sear and magazine release in a SillyButts' Low Hardware Talon Claw U. And also for Blitz Saya / Blitz Shorta mag holders.