Buy one to fit your car model--read the fine print, it should specify--and get good tie=down straps. Don't leave the straps out in the weather, they seem to become very fragile after just a year or so that way. Anything that you carry, secure it redundantly--safety lines to front and back, and securely lashed side-to-side.
And, um, you get what you pay for--but you don't need to pay a fortune.