Sure thing, here's the source:
https://github.com/daleblackwood/package-delivery/blob/master/Props/Conveyor/Con...
The conveyor has a list of kinematic bodies and a speed. The trick here is that they're kinematic and don't collide with other physics objects (at least, not traditionally). The kinematic bodies are added to the list upon collision enter and removed upon exit. The conveyor belt moves each body in the list deltaTime * speed * forwards every frame.
If you need to add rigidbody collisions, I'd recommend using a fixed contraint or axis lock and moving it using an velocitychange of deltaTime * speed * forwards instead.