Guitar string fid.
I was following this thread and making my own continuous loops. My DIY fid is a guitar string. I started thick, thinking a low E would be good--that was a mistake. A "G" or perhaps a "B" from a heavy set functions best. Stiffest, but still thin. For non-guitarists, you could use any string, but the high "E" starts to get too thin to be useful. "Unwound" means a plain wire, not one that's wound with bronze in a helical fashion.
I suppose you could improve on the design with a dowel handle covered in shrink tubing... but my guitar string, doubled, rolls up nice in the Amsteel bag, ready to use.