Thats a good idea with the shower rod. But if you pay the extra $18 bucks for a cargo bar, you're paying for a thicker tubing that is way more heavy duty than a shower rod. You also have a ratchiting action that allows you to secure the bar tighter against the bed walls than you can ever get with turning a shower rod.
I got a cargo bar and I hate it. That thing never worked! It never held in place. The only thing it does is take up space in my garage. I went to the Home Depot and bought a piece of wood and cut it to fit the notches in the bed. Works great