Hi K1124,
McMaster-Carr sells some spring stock (part# 9665K46) which was 20" long, enough for two springs for $15. Attaching the upper loop to the bolt shaft was a challenge on the 21sc because there's no access except from below. Instead of using a loop bent into the spring I found an old luff shackle and used it as the hook. I've been using this for several months with great success. Let me know if you have any questions on it.
https://photos.app.goo.gl/PX27qSWGr7USZxwV7In regards to shock cord, i'm sure 5/16" (8mm) would work. At minimum I would imagine using one band on each side of the centerboard just like the springs. If it's not enough pressure you could probably double it up.