There really isn't an easy way to do it. All convertibles started life as coupes or notchbacks "which ever term you are farmiliar with" then the top was cut off and the task of adding the top frame mechagnism, motor, hydraulics etc. Most of the bracing is still there that the coupe used. Basically, you would have to find a coupe with a good roof section take the roof off with all the inner bracing and swap it out. It is a very involved process which is why you seldom see it done. Your best bet if you are looking for a hardtop is to find another car. The swap can be done but it is in no way cost effective for the average person.