I'm glad the cable ties are working for you. They're even re-useable, if you know what part of the fastener to press to undo them.

The stitch holders I recommended are somewhat rigid, it's true, but they are plastic, not metal. Apparently I didn't understand quite how you're trying to utilize stitch holders in your project - if you'd be willing to take and post a picture of the current setup, it'd be educational.