The original HDPE slider material that was on the trailer when I picked it up from the PO was virtually non-existent.
I had to replace it with something, and I didn't want to wait for mail order plastics (a bit impatient, I know).

This (the tan
stuff) is cut from a TREX or similar plastic composite deck rail baluster. It was at our local big-box home improvement store. They have deck boards and other geometries also.
Sliced in half longitudinally, the 1-1/2" x 1-1/2" baluster fit perfectly on the width of the metal rails.

The pre-beveled corners of the baluster were perfect for this application.
I drilled out the original pop-rivets and put in long #8 SS screws all the way through the rail tube to the other side.
The balusters were only 31" inches long, so I had to piece them. Isanded all my "seams" smooth when I was done, including the angled down-piece.