Deflector for Vibe available?

[Summer]

I travel a minimum of three miles per day on dirt roads and the back of my Vibe is ALWAYS dusty. You might think that a twenty miles of highway driving would clear it off, but no. Does anyone make an air deflector that attaches to the top of the hatch to divert a bit of air flow to blow that stuff off?Also, would said deflector decrease miles-per-gallon much?Paying the price for rural life, Summer