Joe, I mounted a stop to the edge of the left wing and a post on the sled as shown in the first pic. The post could be a problem for some depending on how you store the sled. I dimensioned the sled width so it would fit on a shelf under the crosscut table when stored, so the post is not a problem.
I did not add any plexi over the top. The two bars were added later to help in lifting it in & out of the shelf – the sled is on the heavy side.