I don't know anything about the Sonex kit but do have a few other thoughts that might help. Yes, the cowl can handle the heat. I've used a fence type baffle with the cowl as the top of the plenum with no adverse effects. My latest arrangement though is tight fitting boxes and man, do they work! With the fence deal I saw 375 F in cruise and now it's 275, measured in exactly the same place. A whopping 100 degree reduction. Maybe too cool but that's easy to fix. My inlets are 6 sq. in. per side. Offhand I'm not sure what the outlet is, probably 15-20 sq. in.
The tops of my boxes are about a half inch above the fins, not right down on them, but on the back side of the rear cylinders they wrap tight to the fins all the way to the pushrod tubes on the bottom. In front of the front cylinders there's a wrap so the air can't short circuit by that route. The wraps are made of plain old fiberglass with PE resin and are withstanding the heat.
My oil cooler, a 72 row plate type, is under the engine with about 3/4" clearance to the case. The air comes in the smile and into the gap, then down through the cooler. I have a summer smile that's 5 or 6 sq. in. and a cool weather one that's half as much. The oil stays below 210 in the summer and in the fall I have to partially plug the smile to get it up to 180.
I'm not trying to steer you toward boxes, only letting you know that they do work. But so does a fence and it has the advantage of easier maintenance.
One other thing to look at is the gaps between the fins. If they are clogged with flashing, take some time and clean it out. It makes a difference.
Ed
