Maybe, but there are a lot of possible leak sources that could help with what you have. Post some pictures and you will get a lot of better informed responses.
Some data on your flight conditions and CHT and oil temp will be a good start to convey what "too hot" means in terms of numbers. Airspeed, ambient, throttle, mixture, oil, cut, climb etc would be ideal parameters but what ever you have is a good place to start. After all, installing a plenum will be $$$ and Hrs, so let's make sure it is well invested.
“I often say that when you can measure what you are speaking about,
and express it in numbers, you know something about it; but when you
cannot measure it, when you cannot express it in numbers, your knowledge
is of a meager and unsatisfactory kind.”