well, my suggestion, for the arm itself, Buy a guitar string (either the low E string off a guitar or the A string off a bass) and, cut off the right length you want to be the arm, then do it again, so you have 2 pieces of string.
Glue them together and then coat them in glue (to fix them in the position you wish), this gives you a tentacle style arm (think dr octopus).
I've seen it done before in an Iron warriors army. It stops the arm looking bulky and heavy.
The Iron warrior with this design in question was the Lord who had 4 of these arms.