This may be a dumb idea, but how about putting a volicty stack on the carb, extending through the outside of the cowl? That would keep the leading edge clean.

I'm fighting a cowl now myself. I have built a Great Planes Super Aeromaster. The stock cowl was built up balsa with a plastic nose on it. I did an awful fiberglass job on it so I now have a Fiberglass specialities cowl on the way. I know I'll have to cut a hole in the side large enough to drive a truck through. I'm using a Magnum 91 four stroke with a pitts muffler. About half the engine is going to be outside the cowl and a big chunk of the end of the muffler. I've butchered the wood cowl for a pattern to cut the fiberglass one. I sure hate to screw it up. I'm looking forward to some real answers to your question. I'm sure I can use them also.

Don