I'd probably go with the .25" phenolic open spacer like this one here.
https://www.summitracing.com/parts/s...BoCyJ8QAvD_BwE
If the .25" spacer doesn't provide enough heat shed from the intake to the carburetor to stop your issue, I doubt that a 1/2" spacer would either.
I'm assuming you're using E10 fuel, straight out of the pump? E10 is simply more volatile than pure gasoline and boils at a much lower temperature. Depending on your specific altitude, boiling points for E10 can be as low as around 170 degrees. With under hood temperatures climbing to around 200 degrees or more, as soon as fuel stops circulating in the carb, it's only a matter of time before the ambient temps sink through the carb bowls and boil out the fuel. No heat shield or spacer is going to protect against that condition.
I'm not certain what your fuel setup is, but this is why I believe carb'd vehicles that see regular use and pump fuel need electric pumps in the tank. You can prime the bowls before starting because they will be empty. Or, you just understand that if you're using a mechanical pump, you're going to need to use the starter to get the bowls primed.