Would it be possible to get one of those Saris racks? I know they make some that supposedly fit on all models of cars, even those of us with small SUVs with huge flat backs. Although if your Rav4 has a wheel back there, that might be tricky. You might also be able to pick up the necessary components for a roof rack for a couple hundred and be able to install it yourself (this is what I am going to do for my CR-V).

I agree with Goldfinch... sounds too risky to just strap it down. I'm super paranoid about scratches on my car and my bike.