Two pieces of 3" tubing for stops...weld one standing  up at the edge of each bolster.  Weld imilar pieces on the opposite side, standing up, with a piece of 4" pipe welded on top of them.  Done.  

This one is a little different, it was built from something someone else had already made that didn't work very well. 

Thought I had some better pictures, but can't find them...and it's time to get ready to go now.