Yes, "pancake" simply does not work. The effect differs widely between carbs and between engines for different reasons, but to make the air cleaner housing transparent to the venturi (on/off has no effect) it should be big.
There should also be radial space around the air-horn to about 2.5 × the ID (45mm needs 4.4") so that any wave pulse is completely exhausted inside the housing and doesn't echo off the inner walls.
