OK, Chains. I can't find an entry in the form and material bonuses for them. Is there one?
Based on Rope or cord
+4 restraint or binding
What else is inherently chain like?
I'd be tempted to knock the restraint up and drop the strangulation entirely. After all putting someone "In chains" is well established, but strangulation via chain isn't (although it'd be brutally effective).
Maybe just like a slightly weaker version of shackles
+5 restraint or binding