I removed the heater for the last long summer trip and covered the footwell vents with a sheet of alloy. Made a difference simply not having a hot box of water above your knees.
If you are thinking of ducting air in, take care where you take the air from, if in doubt try driving with the pedal box cover removed on a hot day, the draught may be impressive but the air has gone through the radiator and past the engine, pretty unpleasant.
I was thinking of taking a tumble dryer hose (as suggested) but running it inside the engine bay and round to the front of the radiator, not perfect but cheap and removable.
The ultimate must be to make up a simple box that mimics the external shape of the heater box picking up air from the "heater louvres" down through the hole where the heater was.
You could then go as far as you wanted with adding fans and air filters.
Nick