I have the alternate air intake kit from Vans, but don't like it all that much. The Vans part serves it purpose, but I have other ideas. The purpose of the alternate air door is to provide intake air to the engine in the event the filter become clogged with ice or other forign material. On a Cessna, the carb heat serves this purpose since the carb heat door is downstream of the filter. The downside of the Cessna design is that anytime the carb heat is on, the engine is receiving unfiltered air.
The Vans system has the carb heat upstream of the filter so the engine gets filtered air even when the carb heat is on. The downside is the possible need for an emergency alternate air door. The Vans door is a on-shot system that must be reset on the ground. I played around with making my own version. I made a gate-type valve that can be actuated and reset from the cockpit. The plan is to fiberglass it to the FAB so there are no fasteners that could end up in the engine. Even the bolt used for the actuator is captured when the door is open or closed. I still need to trim the door and rivet the assembly together.
I haven't decided if I am going to install an alternate air door, but it is here if needed,
