The tensioners themselves are made out of a bicycle part, an aluminum shaft threaded with a die on both ends, and a couple nuts. The bicycle part is the thing that bicycles use to keep their drive chain tight against the rear shifting sprocket. (
Link)
It's basically a rubber-ish roller with a groove in the middle and a couple of ball bearings to roll on. This sits on the aluminum shaft, which in turn is double-nutted onto the frame on slots which are somewhat visible (in this picture, the bolt just to the left of the top end of the cutout for the tire. the inboard bolt is not visible from this angle).
If you have any further questions just post.
-q