There has been an interesting thread on ram air and filtering on the RV list. Here is a link to a clever looking solution:

         http://www.lazy8.net/intakesystem.htm

        Looks simple and lightweight, but I am not quite sure where you would mount it on a rotary. Also, it doesn't have a bend in the path to drop out bugs and big chunks when the valve is open. This is easily corrected, however.

        and a more traditional approach

         http://www.rvproject.com/ramair.html

        Again, no "bug bend".