Or just don't add the alt air box. I didn't build it in and since I'm VFR, I see no reason to add it if you are going to be clear of ice....There has also been discussion of the alt air door on the FAB, and my read on this was the the standard alt air door is not so well-designed. I ended up building my own - a sliding alt air door that can be opened and CLOSED from the cockpit rather than the open-only Vans version.
greg
Or just don't add the alt air box. I didn't build it in and since I'm VFR, I see no reason to add it if you are going to be clear of ice.