I do not use the tin that would run across the exhaust port.
Reason is that I'd rather have cooler ambient air flowing over the exhaust valve area than the preheated forced engine air. These engine were designed for stationary use, hence the cooling fan and duct work. At 20+ mph, the cooling effect on the exhaust side of the engine is more efficient than the shrouded air.
Have you ever seen shrouding on a 4 stroke motorcycle engine?
I have over 630 miles on my 4 cycle and no burnt valves or loose exhaust seat, so I' guess I'm the proverbial "dumb newbie"
A more credible concern for those of you who have 4 strokes is to change the oil and adjust the valve
lash regularly.
Just my opinion....