Greg,
From what I have seen most people make them out of some PVC pipe and foam rubber. They take about 1/2 hour to make and take up heaps less room as they are hollow and light. so can fit over your boom in the trailer box.

Take 4 x foot lengths of 90mm PVC pipe and rivet them together internally x 2. It helps if you put a washer over the rivet head on the other side to prevent the rivet pulling out of the PVC.

Get some 5mm sticky back foam to wrap over the top and then add some elastic strap with a hook to hold them on. Total cost about $15 and mine have been going strong for 5 years.

Luke