Mmnnn.... Are there no aftermarket GRP jobs around? MX5's Hardtops proliferate in abundance (even with heated rear screens). The windscreen clips and the cover locks seem simple enough but some consideration of the anchors behind the doors could be a problem I suppose.
BMW and MB SL jobs come with storage systems for the cash.
Admittedly the hoods are superb and I can use a tin top when it is raining, but the option would be nice.
I went out this afternoon and almost departed with the hood down, but just past the next village the wipers were on!!!