Been using cargo carriers for over 20 years now! 50 bucks on sale at Tractor supply is where I buy mine from. And don't forget to pick up two ratchet straps. They hold anything down REAl secure.
They come in handy for just about anything you can think of that you might need to take with RV'ing or that doesn't fit in the bays. I've even moved washers and dryers and couches and beds for my friends with it!!
I now have it attached to my toad I pull behind my MH. Don't leave home RV'ing with out it. :C

I like the lower side model best. It allows you to add any size or type of items and tie them down more securely than the high walled ones.
Be sure to add reflective tape to the sides and back or you will find people running into the carrier in the dark when parked at a CG or even when driving down the road.