The cowl took most of the initial brunt force of the field overturn 2 years ago but was beautifully restored by Roger (Doggett) a master craftsman at Priory Farm.
The repair had involved beefing up and neatening my slightly squared openings i to more curved and streamlined affairs
This, in turn, had slightly unbalance the 1/3 to 2/3 intake to exit ratio for cooling.
Cooling is most definitely a black art ! But generally you start with intake hole surface area measures ..and then make sure your exit areas are twice that ..1/3 – 2/3
As they had altered ..and as I was so so keen to gly post the major repair .. I did what my good and decent engineer would do .. black gaffa tapped up the orifice, clear prop and flew !!
Now ..10 months on ..(ok thats a bit longer than it should have taken) .. ive changed to add a thin metal gurney style plate
The theory is, that the slight (and yet to be adjusted) lip on the metal creates a drop in pressure behind the plate and so aids the exit of air meaning your hols probably dont have to be so large !
Ive started with a very small bend .. as it just flat gaffa tape before !
Haven’t used the old metal cutters since the build. Useful to make some small backing plates for the heat resistant rubber flanges where small washers just aint up to the job.
Jersey Sherwood Ranger 
Hi David , the Gurney has a double effect, as it creates a vacuum that speeds up the flow of hot air extracted, which in turn, draws the cold air in, which reduces the frontal pressure region, thus reducing effort required to maintain the same speed.
lt’s not by much, but on a long flight, it may squeeze some extra distance out of a tank of fuel ⛽🙂.
Kind regards
Nigel
I flew her Priory to Little Snoring Friday .. and temps seemed fine although ine CHT does start to warm when in a powered climb …something to experiment with .. its the front right ‘exposed head.. so tricky to shroud