RE: Baffling a Round-Cowled Plane
With a 30% model, I assume you are powered with a single cylinder engine. Silversurfer is correct insofar as the very best cooling method, which is to leave no other place for the cooling airstream to go than through the cooling fins on cylinder, head and close to the rest of the crankcase. But coming up with that configuration is an awful lot of work, largely of the cut-and-fit variety, which is onerous as heck on a one-off prototype. And very possibly not necessary.
I'm flying a Sukhoi at the moment, round cowl, single cylinder engine, sidemounted. I did what RichardL suggests, which was to cut a piece of 1/8" lite ply in a circle just bigger than the backside of the cowl opening. From the circle I cut out a center circle just smaller than the diameter of the spinner. Then I cut an opening tangent to the top & bottom of my center cutout and extending horizontally to the outer circumference, giving me a slot maybe 2" wide and 3+" long, positioned right in front of my engine cylinder. So my incoming area is maybe 6-7 sq in. My exit area is more like 4"x5", or 20 sq in. So I have about 3x exit area relative to entry area. Engine is very happy, even at static full rpm the head shoots ~250˚ on my IR gun.
As silversurfer says, the incoming air will take the easiest route to the exit area, avoiding as much as it can the "friction" of rubbing up against a hot engine. But if my simple installation works OK from a practical point of view, then I save myself a lot of work by leaving it as it is. If I were getting high temp readings, sagging power, deadsticking, or bearing failure, then I would lay in a couple more pieces of lite ply at right angles to my cooling slot, to carry the incoming air right up against the cylinder (with maybe 1/4" clearance). This would force the incoming air to stream directly at the engine, rather than giving it room to flow around and away from.
A superefficient baffle would be a vertical plate blocking all flow through the cowl, with an opening cut out matching the front view of the engine/mount, leaving maybe 1/4" all the way around the outline, the plate positioned maybe just in front of the sparkplug. Getting the outline would not be so hard, and getting the outer shape to fit inside the cowl at the selected location might be a little harder; but getting the cutout positioned properly on the baffle plate before gluing it in, that would take some cutting & fitting. Work it out with some cardboard stock, and it's doable.
But to be realistic, a simple air dam is a good start, and may be all you need. If you need more, then make a small chute from the dam opening to the engine, to keep the incoming air from simply bouncing off the engine and heading directly for the exit. If that's still not enough, or if you just like to take on a challenge, do the tight outline.
As to how much air you need--surprisingly little: I have a flying buddy master modeler, who used to make test models for a full scale aircraft maker. He spent years in pylon racing, where they developed something called the "compression cowl," which was super slippery on the outside, and encased the engine cylinder very closely on the inside. The cooling air intake was a tiny slot, maybe 1/2" wide x 1-1/2" tall, with a flared intake (which served to compress the incoming air). The slot led to a cylindrical chamber that fit around the engine with the typical 1/4" clearance. The exit was a narrow vertical slot on the opposite side of the chamber from the incoming, which led off in two directions to exit slots on either side of the cowl. This was for a .60 size glow engine. It looked like not much air at all, but it was compressed to some extent, and none of it was wasted.