IIRC Richie's car is basically a cab body draped over a tube chassis (or semi-tube chassis) and the "folded top" is basically a canvas covered wing. You can hardly call it a sleeper but it is a great looking car.
A super cabrio with a canvas cover over a folded top would probably be much more stable at speed than it's hardtop counterpart but going 100+with an open top would be a bit noisy