Clyde,
The permanent folding mechanism with a lined mohair hood would be the way to go, otherwise it will look nasty. But surely it must cost the best part of £10k to do it well. My Aston Volante had a lined hood and that cost £2.5K to replace 6 years ago and that was just cloth and labour. But you have to cut and finish the body, produce a custom built rail for the top of the windscreen that the hood can lock into. You'll need a custom built frame, a new petrol tank and something to cover the area where the hood folds down when the hood is up. If you can even get the hood to fold down into a recess behind the rear seats, which I doubt because the car is too narrow to accommodate the sides of the frame mechanism. You might find you need to remodel the rear seats.
And don't think you won't need air con. Convertibles get even hotter than normal cars because the dark matt roof covering absorbs heat from the sun far more than a gloss painted metal roof does, and there will be plenty of times when it isn't desirable to have the roof down.
Also I can't see it adding value to the car, if anything I think it will reduce the number of potential buyers. Personally I would never buy an aftermarket chop top.
Just go out and buy a nice Stag for £10k
