I had a blow out recently on my bp trailer that ripped the fender off.  I ordered all new parts, but didn't think to order the 15 inches of running board that acts as a brace on the rear of the fender.   I have since been searching, but not having much luck trying to locate a one.  

Any advice? 

Call Sundowner customer service. Measure the trailer for the missing piece.  Measure from the outside edge of the fender to the end point  (the diagonal) Ask for running board of that length.  You'll have to cut the piece for the angle needed and weld / rivet on.

How do I know this ??