Consider the above image.... introduce an incline of 10-15 degrees on the carrier part of your robot. that way when it will be climbing, load will still be almost parallel to the ground... when descending , the load will be stopped by the front rim.
(yes i drew it on paint, and i'm not much of a paint pro
on a side note. 17 degrees is not much of an incline though, if there is sufficient friction between your load and robot surface, it won't slip.