A nice tidy spot, that... but 120C in October doesn't bode too well...
EGT seen over back of panel creeping up to 700C; Oil temp 110C; Oil pressure 35 psi; Boost 12 psi, 14 at times (10 minutes of 2nd gear mountain climbing, would just about pull 3rd between hairpins)
1.9TD AAZ with IC, 2.5" exhaust; Garret T2 with wasetegate bleed; 19 row oil cooler up-front... Doka circa 2 ton AUW
OAT was not high though, at a guess we're about 5,000 ft in Andorra..
As you say, engine compartment needs to be blocked off and a big fan, I tried that spot with a 10" fan (in floor where your cooler is) all sealed pulling air through a pickups tunnel, so not directly comparable... my cooler was in the tunnel leading forward, but far too small, had good flow through it, but no great shakes, maybe 5C max reduction...
Air will not be coming down that tunnel at 50~60 mph, so at the moment you have mainly radiative cooling, about 2~5% of convection possible.
Smoke tablets can be useful, from plumbers merchants... after all...
Who knows where the flow goes... Not me
Yet