I use the rubber ends for the 2x4" and carry 2x 7-foot boards in my ama. I don't tie up often other than quick stops for fuel/water/pumpout, but when you need them it's great to have.

I don't think 3" PVC would be as rigid, and I don't think hanging PVC pipe vertically is a good idea because you'd have high point loads on 2 hard surfaces in contact.... it would scratch your topsides.