I did the same thing with PVC...determine what length you need to accommodate your rods....go to the hardware store and they will generally cut it for you if you wish, then glue on some pvc caps and bungee on to your roof rack....I wrap the rods up in a blanket before putting in for a little more protection....
I use 3" PVC pipe with end caps. Then I used interlocking hose clamps. One goes around the pipe (3" opening minimum) and a second (about 1.25" opening) is slipped through the first clamp. This makes it perpendicular to the first clamp and in line with the rod tube; this is attached to my homemade knock-off Yakima rack. It seems pretty solid and total cost is under 10 or 15 bucks.