KORE makes a spring loaded BMX/ DOwnhill tensioner that mounts on the frame, and even works with 24 speed bikes. I'm going to try it for MB.
This Kore tensioner looks nice but would not work on my build. I can't position it to tighten the chain without being where it would hit the wheel.
$25 shipped USPS to 48 con. states.
mohdoggies @ y!