I did see a plastic tensioner some while ago that would work for you. I
looked like a thin walled plastic sprocket that sits between the two chain
runs. It is compressed to an oval shape and pushes the two sides of the
chain apart.
Hope I have managed to describe it OK.

John


You could also make a floating tensioner like this
http://www.tonysbuggypage.btinternet...tensioner.html

John