I have the same set up (RV4 with O-320). I just bought a carb heat muff from Vetterman for $65. I have the Vetterman 4-pipe exhaust (non-crossover), and strapped the muff to the #2 exhaust pipe.
As for the flap....the instructions for building the filtered air box (aka "FAB") explain how to make one. If you could get a copy of the FAB instructions from Vans, that would be a good place to start.