Possibly the best by far means of ventilating a S2 or 2a I have seen was done by a mate of mine down south. He simply routed some flexible hose from the front of the vehicle picking up from the bottom of the "T" shaped grille (I think he relocated the horn) and running along the mudguard inner and fed in through a hole he had cut through the top of the footwell where he could control the flow by a simple hand operated open/close flap. Feeding from the radiator mount would probably be just as good as the air is trapped and forced through the radiator opening. This system can be applied to both sides obviously.
The beauty of this system was that the cooler air was able to be directed close to the transmission tunnel which is usually found to be a major hot spot, and the one spot that is bypassed by the door type vents.
Regards
Glen