I've used an old shipping cardboard tube with duct tape to bring my freshwater rods back and forth between the Sierras in the Xterra and even on airliners to New Zealand. Hadn't thought about using PVC pipe before, but that would be a little more expensive and whole lot more sturdy. You could even thread just one end cap and fabricate some sort of removable mounting bolts.
I think I'll build one for my salt water surf rod so it's easier to store in the garage as well.
When Hell Freezes Over, I'll Ski There Too!!