I'm not any thermal engineer, although I do have a thermal imager, I suspect that the felt heat is not so much from hot air flow (which much of it is) but from radiated heat. airflow and radiation shielding is required. Even with that, I have found it necessary to wear protective clothing, particularly where the knee and thigh enevitably rest against the tunnel.
in the end, this is a race car; creature comfort was not a consideration.
Some things to consider are that the early 7s and perhaps the majority of 7s made by Lotus and early Caterham were RHD with left side exhaust. Considering the average climate of northern European and UK markets, with lower atmosphere and road surface temperatures, cockpit heat may not have been a major complaint.
The majority of sufferers seem to be the drivers with the exhaust on the drivers side.
In that the 420 is a RH exhaust and you are most likely considering a LHD 420, you may not have too much of a heat battle with the headers being opposite side of the feets.
It is not uncommon for drivers to need to wear the slimmest shoes they can find and racing shoes made with Nomex or fire resistant materials is an option. Perhaps they should be a requirement.
The seven you get will have its own particular set of niggles to work out. Thats part of the fun of having one. You get to work out solutions to problems quite frequently. I appreciate the advance effort of research here. The 7 is going to be what it is. Like a motorcycle, you sit on top of a machine that was barely designed to accommodate human beings, and you eventually learn to stop burning your legs, or you get used to it 😉.
I don't want to derail this thread, but my burning question is, why are you set on a 420? That new car smell is nice, but there are 50 years of Caterhams to choose from. You can get your training wheels for half the price.