Here is a picture of a simple setup,which we did for the pipe tunnel on a YAK
It is made from Depron
Also -look at the location of the louver groups on the cowling -
These keep the big radial cool on the full scale craft.
They also use inlet doors to correct engine temps -as required.
On the Yak -with the huge radial - they place the prop way out in front - to help reduce the prop losses
On a model such as this one -it also helps admit air into th cowl
On my model - all outlet air is via the open "box" under the cowl and thru the pipe tunnel -to a lesser extent.
I measure the temps of my engines and this one -tho it is subject to a lot of pogo stick flying and I crank the engine HARD- it all stays cool - about 190 is the hottest I see at the cyl fins after landing
Look at the aft underside of the fuselage and you can just see the pipe exit baffle .