Joe,
I done the AMT-ERTL kit one time, and modified it into a Convertible by removing the top and scratch building the roof cover along the aft window seal using wet tissue. Once it dries, paint it, and it will be just as hard as the plastic and retain a "material" finish.
Like Chris said, there are no convertible 58's produced, but removing the top isn't that difficult, so it's very doable.
Take care,
Frank