A modified skate board wheel is what a lot of us use. Try for the one pioece style and you'll need a way to cut a groove in it for the chain. I've used a lathe and many have used a drill press with a grinding wheel.
There are tension wheels available from several vendors but most will be just like the one that came with your kit. A good polyurethane wheel will run quieter and last longer than the plastic ones.
You might try using the google search feature, use the keyword, 'tensioner' to see what others have done. There's many pages of reading on the subject. http://www.google.com/cse?cx=partner...ner-33176.html