I did the basic "design" work of the carrier that Paul has on his website. I literally cobbled it togther after taking like three measurements from the TR6 hardtop. After a photo was posted showing a hardtop that I was selling, many people wanted the design specs to build their own so I went out, made some measurements and did a real basic engineering drawing. That drawing is on Paul's site as well I believe. Check out the drawing/measurements and then take a tap to your Spitfire hardtop. They will probably be fairly close.
Paul made some good changes/improvements from my basic design, you might have to as well.