Apparently a block of plastics like Delrin works:<p><a href="http:&#x2F;&#x2F;www.physics.rutgers.edu&#x2F;ugrad&#x2F;205&#x2F;manuals&#x2F;micro.pdf" rel="nofollow">http:&#x2F;&#x2F;www.physics.rutgers.edu&#x2F;ugrad&#x2F;205&#x2F;manuals&#x2F;micro.pdf</a>