Fussy when hot

Might be vapor lock, fuel lines too close to heat coming off the cylinder heads or other hot surface? Metal fuel lines tend to vapor lock worst than the rubber fuel hoses.

I had a similar issue and rerouted the fuel line to make sure it did not sit on the tins.  That solved it.

I am considering getting some heat shield wrap for my hoses.  It is not expensive and seems simple to do.  My engine is not a show engine...although brand new, I dont think it is something that I am going to pop the hood to show many people.  But, I had this problem with my last speedster and I believe it is also the fuel lines getting too hot.

"I'm also thinking of punching a firewall hole, similar to where Vintage place it."

What manf is your Speedster? - CMC/FF build manuals have you cut a 1/2 moon piece of fiberglass out of the engine firewall above the flywheel to give air to the intake fan.  No screen on it - which probably would be good idea. Check out the build manual in the Library (for donating members).

Your "5mm gap" could be a significant cause of excess temperature. I would check around the entire seal and replace it right away.

 Yelp the engine compartment seals are the most important thing to to keep one cool  I even will use a dab of urethate to seal up any tinware gaps and cracks.. Get that stuff right.. I even made a engine self out of sheet metal with the vw channel for the Vw bug seal to fit in   look at the lower right pic