da Rock

A couple of suggestions.

Either block off or place a deflector in the cowl opening that's not in front of the engine. It's going to allow air into the cowl, but the air will flow right through and have no reason to go anywhere near the engine.

"Pressure cowls" are cowls that have all their interior blocked except for very close around the engine. They block completely except immediately next to the engines' cooling fins. They usually only use sheet metal because all they're doing is blocking airflow that would bypass the engine if allowed to.

Cool air is easier to move than hot air. Cool air is coming in the front, and if it runs into hot air, it simply goes around the hot air if it has a way to do it. You need to block those easy paths.

I'm including a couple of pictures of my Sukhoi. It's got a HUGE open cowl. I used 1" thick pink insulating foam board I picked up at Lowe's Home Improvement. It was very easy to work with. I filled all of the cowl that wasn't taken up by the engine and muffler. The foam also made the lousy-made, warped fiberglass cowl round again and gave it tremendously more strength.

You'll note that the opening in front of the engine is still huge. Sorry I don't have a picture from angles that'd show the exit holes, but trust me, they're not greatly larger than the "intake" area. The ratio of intake to exhaust only takes into account the area available to the air to pass at it's most restricted area. You can easily get by with a ratio of 1 to 1.5 if you block off the cool air bypasses. A "pressure cowl" design does that.