I'm not sure where I saw it, but there is a company that makes a form fitting convertible top cover that fits over the entire top and windshield. It's tailored to each individual cars design. Not sure how it's secured.
I know if I had a roadster and I drove it to work everyday, and it had to be parked outside in the sun, I'd probably buy a white cover to protect my top from the sun, birds, cats, acid rain, and prying eyes.