The circa 1990 rubber that had been on my car for some 30 years coped with this quite well, but was a little tired, and when I bought a new one it was considerably thinner.
This made it impossible to assemble the plenum nose, lubed or not, through the rubber without pulling the outer edge of the rubber off the air-box entry.
I knocked up a piece of zinc sheet with an interior diameter (162mm) that was a perfect fit inside the new rubber, and an outside that matches the outer edge of the air-box upper ring (196mm), BUT with a 5mm offset of their centres.
In this way I was able to rotate it until the rubber was under the least possible lateral duress, as its eccentricity means you can optimise the alignment of the plenum-nose within what is effectively a reduced-diameter entry into the air-filter.
I then used the car for 1.000kms with the zinc just sitting on the airbox top, where it sat happily, and effectively, and I think it would have done 100.000km without further attention.
However, I got a new one water-jet cut out of 15/10 stainless plate, that I had powder-coated, and then secured it with one small self-tapper ; you could also glue it with a bead of black SIKA if you didn't trust the seal it was making, but I think that's unnecessary.
If anyone thinks that might be useful to try on their car, faced with a similar problem, they can have my zinc one for free.
