A relay would mount, like a new car, on the firewall or fenderwell. That would keep it cool. The relay carries minimal current compared to the starter switch (a.k.a. solenoid).

I drive my car on many cruises, including hours and hours of stop and go traffic (Woodward). Stock compression 400 auto with air and ps, original radiator, original fan, stock water pump, lots of heat. The ONLY time I had a hot start issue was when we drove to Gary Plowman's house, about 2 1/2 hours on the freeway, stayed just long enough for a thorough heat soak, back on the freeway, then shut it off at a gas station after a few miles. I had to back the timing off about 3 degrees to start it (rather than waiting for it to cool). Starter is at least 25 years old and has not been modified. I start it up and shut it down all the time without issue.