DIY hardtop from a 996

[boxxster]

I think you'd have an easier time modifying the profile of a 986 hardtop to look like the 996 top than making the 996 top fit. Leave the window inset and just add some extra material on the sides so it slopes more gently to the back of the car like this. It won't look the same as the 996 top from the back but you could make it pretty similar looking from the side.