My initial thought is to get a PVC tube with one end capped, drill some holes along the length, and secure it upright with zip ties on the side of my rack.
This is a 4-piece rod, but a 3-pc. works just as well. I have a lot of rods. I've tried a 3pc. in a leather tube strapped to the top tube, but it's ungainly compared to this rig.