Back when 1293 built Ockham, it used long timing belts one of their engineers procured. They made it into a loop using a small aluminum plate that captured the teeth of the belt right at the ends, and then mounted that near the end effector. It appears you can get long timing belt at McMaster-Carr, though it doesn't look to be cheap.
edit: If you look carefully at the big version of
this picture of Ockham, you can kinda see the plate behind a bundle of extra belting in the front of the robot.