Pre-cut soundproofing kits were available for most makes of cars, and if there was not a kit for yours it was possible to buy the material by the yard.
By making a paper template it could be cut to fit, using the appropiate adhesive.
I would have thought this material was still available through upholstery/fabric trimmers etc specialists.
However I could never understand why one would want to make an open sports car quieter, surely much of the joy is to hear the crackle of an exhaust and the roar of the air intake gulping!!! - unless you always had the hardtop on and needed peace and quiet for long journeys.
Also think of all the added weight on top of an already heavy car.