FYI, there's a whole bunch of research done by IAM in the early 90s. Unfortunately not (apparently) in public domain.
Seems daft there's no formal guidance (e.g. H&SE), given the habit of bolting a greenhouse to the front of the airframe, then dressing in overalls before sitting in the sun.
In unpublished work

, IAM found mid-summer Germany was enough to terminally fry the pilot

in less than 2 hours for some workloads, even after acclimatisation

. Best answer was the liquid cooled (15 deg) suit, with an attached cooling skull cap worn under the helmet

, which also supresses sweat. Pipes, plumbing, escape and cooler weight were all reasons why this isn't done now. That and looking a prat queueing at ATC with that lot on.
Perhaps an opening sunroof would help, given the big cooling fan on top (Brylcreme might be needed

).
Nimby