I am still assembling the H-9 P 51 and just getting to the muffler installation. It will be completely enclosed exactly like all of my other P 51 models. The muffler is steel construction I welded up myself. I got tired of paying $150+ for aluminum exhaust that sometimes cracked after a few flights. It is heavier than the hobby stuff but I need weight in the nose of this plane anyway.
The carb air filter is behind the firewall so it draws its air from the inside of the fuse. Some openings are required in the belly scoop and radiator exhaust to supply air. I glue a balsa block in the carb air opening under the spinner, then route out an aerodynamically shaped cooling air passage back to the upper cylinder. This keeps all the incoming air directed at the hot parts. The next step is a shaped cooling exhaust that is notched in the lower firewall. A wedge-shaped passage allows hot air to flow straight back and out under the fuse belly in front of the wing. High speed air flowing pass the cooling exhaust creates suction that helps pull the hot air out. I have never needed a dam or spoiler here so I don't bother with that.
I drill out most of the scale exhaust to the inside of the engine cowl, they are angled back into the airstream so there is a natural suction to remove heat from the upper engine compartment.
Most modelers just cut holes where it is convenient and trust the opening to do all the work of moving hot air. The heated air off of the cylinder and exhaust system has to have some space to expand two or three times its volume and then naturally keep moving without restriction out of the plane. A simple hole in a flat surface that is in line with the air stream won't guide hot air out and usually creates a restriction to air flow. That causes the overheating and dead stick landings.
Modelers want to keep it simple and not spend the time and labor to make it work. It is, after all just a toy. I had some extensive aircraft mechanic training both in the US Army and later for Bell Helicopter. I am used to seeing air intake and exhaust handling solutions in full scale aircraft and helicopters. I try to keep the scale models as simple as possible but a certain minimum of design and construction effort is necessary in order to have a nice flying day. In my military job as a crew chief I had to pay attention to the details so that others would not pay for a mistake with their lives. And I was along for the ride with them. I didn't have anything to do with the design of the aircraft I flew in but I got to take them apart every day and just got used to the way it needs to be. I flew up to 14 hours each day and pulled maintenance inspections all night with my gunner. When I needed extra manpower they got line mechanics out of bed to help us do a week's work in one night. When that happened I had to inspect all the other's work too. I got to sleep every time the pilots shut it down to go to a meeting for their mission orders (usually 10-15 minutes unless we were in a hot LZ being shot at).