As Dave W has said, additional heat insulation to the bulkhead, and if your car has cats, to the footwell sides and floor.
My plus 8 has A/C, which is very effective at keeping feet/legs cool and if the hood is up, the cabin. But the Aero Plus 8s have a revised heater box with an electrically operated external hot water valve, so it is possible to completely shut off hot water from the heater box.
Also, as has been said, make very sure that the foam between the top of the heater box and the bonnet so that hot air from inside the engine bay doesn't get into the heater box.
Another, rather more complex solution but one which would not be impossible, would be the footwell vents as fitted to 1950s Jaguar XK 120 and 140 models. They scoop in external air.... Not much fun in the UK as they also scoop in water if not shut properly.
