I have a '04 HCH that I put a hitch rack on, I realize you said you didn't like them but I thought I would add what I could. I have had it on for about a month and in just local driving, with at most 2 bikes, my mileage does not seem to be hurt. With the Prius I would think since the car is larger than the Civic, then there would be less impact for you. You might want to look into a hitch rack since they are easier to take on and off and the hitch receiver has little impact on the car.

Billy