Scott Keighan writes:

Has anyone come up with an off the shelf or kit for the induction system....

Scott, have you looked at the "automatic" alternate air system specified in
the IV construction manual and found it deficient?

We've built two IV-P's to the construction manual specs, and although we've
not tested ours (birds not yet ready to fly), I (unintentionally) tested
this design in a friend's IV-P and it worked as advertised when the air
filter apparently iced up. If you use an adequately sized filter, the
basic/primary air induction system should provide a ram feed to the engine
when at cruise airspeed--and if the pressure in the plenum goes negative
(due to filter icing?) the alternate air door will open to feed warm air
from around the engine to the engine air inlet. Seems pretty simple and
elegant to me.

Bob Pastusek