Go to mcmaster.com and type in 5973K1. My idea is based off of this but has a different way of tightening it and is superior in price and quality. Here is my final design of it:
We have been using Mcmaster Carr tensioners and chain does not get locked into the tensioner. UHMW is a very good plastic for this application and the side walls are usually never hit due to the smaller side walls. It is fit for both #25 and #35 chain.