I have gone to wearing long pants when out on a blat to prevent scorched calf. This is after adding header wrap and applying thermal barrier material to the firewall and some of the tranny tunnel.
My guess is the system is pressurized during operation. The cap on the t-stat housing should have a spring to hold the sealing surface down. A new cap rated at 10 to 13 psi is an inexpensive experiment.
Another FWIW... I don't recall if this issue has been addressed and that is that possibility of vacuum leaks.
You can use carb cleaner to hunt with. A spray on an area like intake gaskets etc. and listen for a change in RPM.
Happy hunting.