Backdraft shutter on extractor fan

I put a backdraft shutter onto the fan in bathroom.

It was flexi pipe out to the soffit. I got a short length of 100mm pipe and screwed the shutter to one end (screws don't impinge and is in correct direction) and to the fan at other.

Is there anything that is used to seal the pipe against fan as there is a bit of a gap. At present the extracted air seems to leak out at bottom between pipe and fan.

I could wrap the joint with tape, but wondering what or how those in the trade do the job.

Here is my install:

I do get a hear a bit of flutter from the shutter which I think is because it is located just before the flexi pipe bends downward into the soffit. I'm thinking when wind blows up the soffit and into the pipe, the air is hitting the top of the shutter and slightly opening it.

Should I have installed this differently and is there anything other than tape to seal between pipe and fan?

Thanks.