Car Cover - Use a Fitted or Generic cover for a W107?

[London380sl]

I have a 84 380sl. I'm getting tired of washing my car (it doesn't help my neighbours kid me about either) so I'm thinking of getting a car cover to reduce this task. I'm not sure on whether to a pay the additional amount and purchase a fitted car cover or get a generic car cover that fits a number of cars which are approximately the same size.

Also, I'm confused how a custom fit cover would fit a covertible. I presume they are fitted on the basis of the top being raised. If so, how good a fit do you get when the top is down?

Any input from car cover owners would be appreciated.

Thanks