removable hard tops are a cool idea, however you have to think of storing it in the car in case you happen to drive into a rainstorm. The trunk space is not nearly enough to house a simple hard shell that has no folding joints, but folding joints means more time in R&D, and obviously more funds in the same areas plus material. To humor this hard top idea, I have done a little searching for you.

new top....hardtop: 240SX Convertible Forum: Nissan Forums / Infiniti Forums - NICOclub
Custom Hardtop protype: 240SX Convertible Forum: Nissan Forums / Infiniti Forums - NICOclub

The search function on Nico is honestly better than many other forums, mainly cause its powered by Google.